could even solve two of the biggest mysteries in modern physics.
Around 27,000 lightyears from Earth, in
the very center of our galaxy is
Sagittarius, a star, a massive and
extremely compact object shrouded in
dense clouds of interstellar dust that
by its very nature we cannot observe
directly. The first clues of its
existence were found in the 1930s when
Carl Jansky detected an unusual radio
signal coming from the direction of the
Sagittarius constellation. At the time,
no one knew what it was. Black holes
were still considered mere mathematical
curiosities and hypotheses about what it
might be buzzed around the scientific
community. Could it be clouds of star
clusters or perhaps remnants from a
supernova?
It wasn't until 1974 that it was
identified as a single compact object by
Bruce Balac and Robert L. Brown. Brown
later named it Sagittarius A star. The
theoretical groundwork for the existence
of black holes had been laid in the
intervening decades, and it didn't take
long before two and two were put
together, and speculation grew that
Sagittarius A star was likely one of
gargantuan size. As observations
improved in the 1990s, astronomers were
able to see stars orbiting an apparently
empty region of space at astonishing
speeds. Since those stars must be under
the influence of something millions of
times the mass of our own sun, they
became the strongest evidence we have
for a super massive black hole. The path
of these stars, termed s stars, became
remarkably wellmapped. They are the
closest known stars to Sagittarius A
star and move at speeds of up to 24,000
km a second which is around 8% the speed
of light. One of them S2 completes a
full orbit in just under 16 years. For
context, our sun takes more than 200
million years to complete its orbit of
the galaxy. By tracking the orbital
motion of these S stars, astronomers can
calculate the size of whatever it is
that is sitting at the heart of the
Milky Way. In fact, the researchers who
did this won the Nobel Prize in physics
in 2020.
the galaxy where two independent teams led by Reinhardt Gensel at the Max Plank
Institute and Andrea GZ at UCLA found
that Sagittarius A star has a mass of
around 4.3 million suns and it's
confined to a region only 23.5 to 25
million km across which is small enough
to fit within the orbit of Venus. Their
Nobel prize was shared with Roger
Penrose who provided the mathematical
proof that black holes are a direct
consequence of Albert Einstein's theory
of relativity. It seemed that
Sagittarius A star's status as a super
massive black hole was cemented.
But detail here is important and the
official citation on Gensel and GZ's
Nobel Prize reads for the discovery of a
super massive compact object at the
center of our galaxy. And whilst the
consensus is that Sagittarius A star is
a super massive black hole, it has not
actually been proven and scientists are
still testing alternative ideas. In
fact, it's never been the only possible
explanation, and other theories have
been around since the first detection of
Sagittarius A stars radio signal. But
the discourse moved to the fringes as
other possibilities, such as a star
cluster, were eliminated as the SAR data
built over the decades. But some ideas
remain including boson star models which
propose that Sagittarius A consists of
bzonic particles,
gravis stars, dark matter cores
surrounded by a shell of high energy
matter and compact cores which are made
entirely of dark matter.
To understand why these ideas have
persevered in the literature, we need to
consider what we don't know about
Sagittarius A star. And it's here we
find one of the greatest unsolved
problems in astrophysics.
How do super massive black holes form?
Well, here's the simple truth. We don't
know.
Stellar and intermediate mass black
holes, which range from a few times the
mass of our sun up to 100,000 solar
masses, form when a massive star uses up
its nuclear fuel and collapses in on
itself. Under certain conditions,
including the starting mass of the star
relative to the remaining core, these
violent deaths leave behind a black
hole. This process is very well
understood and the signatures of these
black holes are well observed. Some
astronomers estimate that there may be
as many as 1 billion stellar black holes
in the Milky Way alone. But Sagittarius
a star is a completely different
category. At 4.3 million solar masses,
it is far too large to have formed from
the collapse of any known star. There
simply are no stars massive enough to
collapse and form a black hole of this
scale. Once more, by definition, we
cannot observe black holes directly.
They do not emit light and photons that
cross the event horizon become trapped.
Their existence is only inferred by the
effects they have in their surroundings.
And this leaves the door a jar for
alternative theories. and an elegant one
hit the headlines in February 2026.
Valentina Kresby at Institute of
Astrophysics La Plata in Boros Aries
studies dark matter at galactic scales.
She and her collaborators statistically
compared an alternative black hole
model, one where Sagittarius A star is
composed of a dense dark matter core to
the observational data of S2 and five
known G objects. G objects are a unique
class of objects discovered in the early
2000s. They behave like stars but look
like gas, visibly changing shape as they
move closer to Sagittarius A star.
distorting under the influence of its
intense gravity. It's thought they could
be compact dust clouds or stars cloaked
in a thick shroud of gas and dust. This
strange shifting morphology would need
to be replicated by any models
challenging the super massive black hole
consensus. And that's exactly what
Cresby set out to do. Her aim was to
determine whether a dark matter core
could reproduce the behavior of S2 and
the G objects to the same level of
precision as the black hole model.
Published in the monthly notices of the
Royal Astronomical Society in February
2026, the results were remarkable. A
dense core of dark matter in this model
could reproduce the orbits with less
than 1% difference to the black hole
model.
What this means is that you cannot tell
the difference between a dark matter
core and a black hole with the
observational data tested. Both have the
same gravitational effects on S2 and the
G objects.
Even more remarkable is that this model
has implications far beyond the nature
of Sagittarius A star. Kresby and her
international collaborators propose that
it solves another huge mystery in
physics. What dark matter is made of.
To understand how significant this is,
we first need to take a small detour and
unpack what we currently know about dark
matter. I'll cover it in summary here,
observation of dark matter. Dark matter was first proposed by
Fritzviki in the 1930s after he observed
that galaxies in the coma cluster were
moving too fast for known physics to
explain. With relative speeds of more
than 2,000 km a second, these galaxies
should have flung themselves apart if
the gravity holding them together was
proportional to the matter he could see.
Something else was adding mass to the
system and he termed it dark matter.
Later observations saw something similar
in the rotation of galaxies. Stars in
their outer regions orbit at speeds that
cannot be explained by visible mass
alone. Dark matter must be providing the
extra mass to maintain these velocities.
And it turns out it permeates the known
universe.
Closer to home, it is thought that the
Milky Way is surrounded by a dark matter
halo, a vast and disused sphere
stretching far beyond the visible disc
of stars and gas. So, in a situation not
dissimilar to the one I described
earlier, we know dark matter exists
through the gravitational effects it
has. And current models suggest that
dark matter makes up about 85%
of the matter in the universe.
But we don't know what it's made of. It
doesn't emit, reflect, or absorb light
in any known way, and it has not yet
been directly detected. There are a
number of candidates in the race to
identify dark matter, with axons and
wimps the most widely studied.
An intriguing paper published in
November 2025 may have found a telltale
annihilation signal associated with
wimps in historic data from the Fmy
telescope. But snags remain and the dark
matter question is still open. And this
is where the team at La Plattera comes
back in. They propose that dark matter
consists of dark fiance, a candidate
that is not expected to produce any
detectable signals at all. It's an idea
that was first put forward by
astrophysicists Rufini and Bonazola in
1969
and it's been fizzling in the background
ever since. Firmians are subatomic
particles including electrons and quarks
with half integer spin and are basic
building blocks of matter. Of crucial
importance for us is that they obey the
poorly exclusion principle. No two
identical firmians can occupy the same
quantum state. In other words, they
cannot be in the exact same location in
space with the same energy and the same
spin at the same time. This is a
fundamental rule of quantum mechanics
and a key reason why matter doesn't
collapse in on itself and why we can't
walk through solid walls. This is a
property they share with the proposed
dark theians. They are posited to be
elementary particles too, but they only
interact with the rest of the universe
gravitationally,
not electromagnetically,
making them dark. Because of the poorly
exclusion principle, dark theians cannot
be infinitely squeezed together. They
push back and resist collapse. So they
could coalesce and under the right
conditions build up huge amounts of
internal pressure. This results in an
ultra dense stable object which in
theory could reach masses similar to a
super massive black hole. Proponents of
this model including the group leader at
Llata, Dr. Carlos Aguas, suggest that a
core of theionic dark matter would be so
dense that it would in every observable
sense be indistinguishable from a super
massive black hole. But unlike black
holes, such cores would not form a
singularity nor have an event horizon.
And that's not all. Fionic dark matter
would extend beyond a compact core,
naturally forming a distinctive
structure that diffused through the
galaxy and beyond to form a halo. Put
most simply, it's proposed that
Sagittarius A star and the dark matter
halo are not two separate objects. They
are two parts of the same continuous
structure made from theionic dark
matter. This is something no other
Sagittarius A star hypothesis does. In
every other scenario, the compact object
and the dark matter halo are two
distinct structures. This is the only
theory that unifies what we see in the
galactic center with the hypothesized
dark matter structure of the galaxy.
It's incredibly elegant and rather
convincing, I personally think. Now, as
we've seen, one of the key pieces of
evidence for dark matter's existence is
how galaxies and the stars within them
rotate. Something is adding mass and
influencing their speed. ESA's Gaia
mission is bringing extraordinary detail
to this picture. Its aim is to
accurately measure the motion of 1
billion stars as they orbit the center
of the galaxy from its inner regions to
the outer disc. And the published data
in 2022 brought an intriguing twist to
this tale with particular importance to
the theionic dark matter model.
Gaia revealed something totally
unexpected at the outer edges of the
galaxy. A slowdown in the rotation of
its outer arms called Kepleran decline.
This presented a problem for the
standard picture of our galaxy. Most
dark matter models cannot reproduce this
observation. But theionic dark matter,
thanks to our old friend, the poorly
exclusion principle, could form a dark
matter halo that is a clearly defined
sphere. It would have a sharper boundary
than other dark matter candidates. And
this would naturally lead to the drop
off in speed observed by Gaia. In other
words, Gaia is not just mapping stars.
It's tracing the distribution of gravity
across the Milky Way. There are other
observations that can show us gravity's
effects in our galaxy. And the most
well-known takes us back to where it is
strongest, the extreme environment
around Sagittarius A star. This
enigmatic image from the event horizon
telescope published in 2022 was widely
described as the first direct image of
our black hole shadow. Again, the devil
is in the detail here. What the EHT
image shows is glowing hot matter
circulating Sagittarius A star bent into
a ring by the object's extreme gravity,
which is consistent with a black hole.
We're also seeing a shadow against the
surrounding glowing gas. In a 2024
publication, Agues and his team
demonstrated that a dense fionic dark
matter core could mimic this shadow
because its extreme gravity would bend
light just as strongly, producing an
image almost indistinguishable from the
black hole scenario with current
instrumentation. As Kresby describes it,
our model not only explains the orbits
of stars and the galaxy's rotation, but
it is also consistent with the famous
black hole shadow image. The dense dark
matter core can mimic the shadow because
it bends light so strongly, creating a
central darkness surrounded by a bright
ring. And if it turns out to be true,
this theory comes with some other
exciting implications.
Thanks to the James Webb Space
Telescope, astronomers are able to peer
deep into the past, imaging galaxies
whose light has taken so long to reach
us that we're seeing them as they
existed less than a billion years after
the Big Bang. Something interesting has
been found in the center of some of
these galaxies called little red dots.
These are thought to be compact masses,
hundreds of millions or even billions of
solar masses in size. And it's very
difficult to explain what they are. How
could something so large form in such a
short time scale? Since their discovery
a few years ago, there have been
hundreds of papers and almost as many
theories that attempt to explain them.
Fermionic dark matter cores fit
remarkably well here too because they
could be established far earlier in the
history of the universe than a super
massive black hole. In the dense
conditions of the early universe,
theionic dark matter would have been
able to clump together to form compact
configurations without needing any other
starting point such as stars at all.
which would explain why we can see
extremely massive objects so early in
the historical cosmic record. But before
I get too far ahead of myself, let's
take stock for a moment. We have SAR and
G object orbits, the GIA rotation curve,
and the EHT shadow. A thermionic dark
matter model can reproduce these current
observations with striking accuracy. But
as the teams are quick to point out
themselves, their theory is not proven
and their work has not shown the black
hole model to be wrong or shifted the
consensus view. It is still considered
most likely that Sagittarius A star is a
super massive black hole. But what
Kresby Arguas and their collaborators
have done is shown that a firmionic dark
matter model cannot be ruled out by
current data and it deserves a closer
look. And beyond our current abilities
to detect, there is a signature unique
to black holes that could help settle
the matter.
around a true black hole. The intense
gravity near the event horizon can force
photons to spiral multiple times before
they disappear beyond it, never to
emerge again. This phenomenon called
photon rings would not exist around a
dark matter core because dark firmians
would resist being squeezed into a
singularity in the first place. The dark
matter core remains a stable, dense
sphere rather than a bottomless pit.
Without the event horizon, light is
simply bent by the core's gravity and
flies away rather than getting trapped
in the endless loops that create a
photon ring. Imaging such a structure
around Sagittarius A star is actively
being worked towards. NASA's black hole
explorer mission aims to launch a new
space-based radio telescope which would
link directly to the next generation
EHT.
Such an endeavor would create a virtual
telescope larger than Earth, but it
could be decades before this comes to
fruition. In the meantime, the gravity
interpherometer at the VLTI in Chile is
being continuously developed. This
interferometer is already tracking the
perisenter procession of S2's orbit,
measuring the subtle changes in its
orbital path with each loop around
Sagittarius A star. The direction of
this shift and by how much could provide
further constraints for competing models
to be tested against. At the other end
of the scale, direct detection
experiments like the neon experiment in
South Korea are beginning to hunt for
dark fian.
Researchers are looking for the minute
traces of energy resulting from a
collision between a passing particle and
an electron. If signals like this are
detected, they could offer vital clues
about the properties of dark matter and
help us understand what it's made of.
What I find so fascinating about the
Fionic dark matter hypothesis is its
elegance. For decades, two of the
deepest mysteries in physics have been
treated as entirely unrelated. The
nature of the super massive object at
the heart of the Milky Way and the
nature of dark matter that makes up most
of the universe. The LLA team and their
international collaborators propose that
these problems are one and the same and
it's compelling. If they are correct, it
would change our understanding of
Sagittarius a star and the universe
itself. It's a bold claim and the
consensus still points overwhelmingly
towards a super massive black hole. But
both are far from proven, reminding us
that some of the most fundamental
questions remain open even at the center
of our own galaxy.