64 Scientific American, April 2022NASA/CXC/CfA/M. Markevitch (x-ray), NASA/STScI, Magellan/U. Arizona/D. Clowe (optical and lensing map), ESO WFI (lensing map)neutrino: the sterile neutrino. Sterile neutrinos are dis-
tinct because they interact primarily gravitationally and
only mildly through Standard Model forces. In addition,
they are perhaps the most popular warm—or at least
somewhere between hot and cold—dark matter proposal.
Another idea that theorists are just starting to
explore is that rather than a single dark matter parti-
cle, there may be an entire sector. Perhaps dark matter
is made of traditional axions, axionlike particles,
WIMPs, sterile neutrinos and SIDM—all together. One
other tantalizing possibility is that dark matter actually
comprises stellar-mass black holes that would have
formed in the early universe. This option has become
more popular since the 2017 detection of gravitational
waves indicated that black holes in this mass range are
more common than expected.CLUES IN THE SKY
in astronomy we are relatively passive observers. We
can choose our instruments, but we cannot design a
galaxy or a stellar process and watch it unfold. Cosmic
phenomena rarely happen on human-friendly time
scales—galaxy formation takes billions of years, and
the cosmic processes that might emit dark matter par-
ticles do so over tens to hundreds of years.
Even so, astrophysical probes of dark matter can tell
us a lot. For instance, the nasa Fermi Gamma-ray Space
Telescope has functioned as a dark matter experiment
by looking for gamma-ray signatures that could be
explained only by dark matter. WIMPs, for instance, are
predicted to be their own antimatter partners, mean-
ing that if two WIMPs collided, they would annihilateeach other just as matter and antimatter do on contact.
These explosions should produce an abundance of
gamma-ray light where there is dark matter, especially
at the cores of galaxies where dark matter is densest.
In fact, the Fermi telescope does see an excess of
gamma-ray light at the center of the Milky Way. These
observations have inspired passionate debate among
observers and theorists. One interpretation is that these
fireworks result from dark matter colliding with itself.
Another possibility is that the signal comes from neutron
stars near the center of the Milky Way that emit gamma-
ray light through the typical course of their lives. Some
astrophysicists favor the more mundane neutron star
explanation, but others think the signal is dark matter.
The fact that there is disagreement is normal, and even
I have a hard time deciding what I think. I am compelled
by physicists Tracy Slatyer and Rebecca Leane’s thought-
ful research showing that a dark matter explanation is
sensible, but in the end, only analysis of more detailed
observations will persuade the community about either
idea. Future data from the Fermi telescope and proposed
experiments such as nasa’s All-sky Medium Energy
Gamma-ray Observatory eXplorer (AMEGO-X for short)
have the potential to settle the debate.
Scientists have also used the Fermi telescope to look
for evidence of axions. Theories predict that when axions
encounter magnetic fields, they occasionally decay into
photons. We hope that by looking over long distances, we
might see signs of this light, offering proof that axions
exist. And neutron stars—the potential confounding sig-
nal at the Milky Way’s center—are actually a good place
to look for dark matter on their own. Some theories sug-BULLET CLUS
TER: Ob serv a
tions from the
Chandra xray
space telescope
show the loca
tion of normal
matter ( in pink )
as two galaxy
clusters collide.
Gravitational
lensing studies
revealed the
bulk of the mass
( in blue ) was
separated from
the normal
matter, offering
strong evidence
for the existence
of dark matter.