Graphic by Jen Christiansen April 2022, ScientificAmerican.com 63But when Mark, Alan and I sat down to check how
Sikivie’s group had arrived at this prediction, we came
to a radically different conclusion. Although we agreed
that axion Bose-Einstein condensates would form in
the early universe, they would be much smaller—the
size of asteroids. Our model also did not give any indi-
cations, in the present-day universe, of what kind of
axion structures we might find billions of years in the
future. Trying to better model how—and whether—we
get from small asteroid-sized condensates to the galac-
tic-scale dark matter halos of today is still a significant
computational challenge.
The same year our paper came out, another group
was looking into other interesting implications of
axionlike particles. A team led by Hsi-Yu Schive of
National Taiwan University published computer sim-
ulations of certain axionlike particles that are often
referred to as “ultralight axions” or “fuzzy dark matter,”
so named because they have a very low mass and would
act like blurred-out waves rather than pointlike parti-
cles. They showed that these particles could form wave-
like dark matter halos with Bose-Einstein condensates
at their cores. Schive’s paper generated new interest in
ultralight axions and raised hopes that astrophysical
observations could detect signs of the wavelike halo
structures we expect.
These days axions and axionlike particles stand
along with WIMPs as some of our best guesses at what
dark matter could be. Another category that is grow-
ing in popularity is a model called self-interacting dark
matter (SIDM). This idea predicts fermion dark mat-
ter particles that have some kind of interaction with
one another—a self-interaction—beyond gravity. These
self-interactions could create more interesting shapes
and structures within a halo than a smooth, spherical
blob. The particulars of the structures are hard to pre-
dict, though, and depend on the mass and other char-
acteristics of the particles. Interestingly, axions may
also interact with one another, though in different
ways than self-interacting fermions.
There is an alternative to WIMPs, axions and SIDM:
neutrinos. Although Standard Model neutrinos are now
known to be too low in mass to explain all of the miss-
ing matter, these neutrinos are real and hard to see,
making them functionally a small component of the
dark matter that we call the cosmic neutrino back-
ground. In addition, a new type of neutrino has been
hypothesized as a companion to the Standard ModelFERMIONSBOSONSCOLDDARK^MATTER^ CANDIDATESCOLDDARK^MATTER^ CANDIDATESElectronsQuarksWIMPsSterile
neutrinosBlack holesAxionsAxionlike
particlesPhotonsBosons or Fermions?
Popular Dark Matter Categories
Scientists have a plethora of theories for what dark matter might be. Most suggest that the missing stuff is made of particles that
have not been discovered yet. These fall into two categories—fermions and bosons.Fermions include many particles
familiar to us, such as the electrons
and quarks found inside atoms.
These particles all have an odd half
integer spin (such as^1 ⁄ 2 ,^3 ⁄ 2 , and so on)
Two popular dark matter candidates,
sterile neutrinos and weakly
interacting massive particles,
would also be fermions
(although there are
versions of WIMPs
that would not).Bosons include
the known force
carrying particles,
such as photons,
which carry the
electro magnetic force.
A hypothesized kind of
boson, called an axion, is another
popular dark matter candidate.
In addition to the traditional axion,
scientists have conceived of variations
known as axionlike particles, with
slightly different properties.Dark matter may turn out to be one of these candidates,
or none of them, or maybe even all of them. Or it may not
be particles at all but small black holes throughout space.