24 ASTRONOMY • MAY 2020
speculate about the kinds of particles
that might make up this substance.
In particular, researchers have long
recognized that if dark matter particles
interact through a force that is approxi-
mately as powerful as the weak nuclear
force (which governs radioactive decay),
then the number of these particles that
should have emerged from the Big Bang
would roughly match the measured
abundance of dark matter found in
the universe today. With this in mind,
weakly interacting massive particles —
WIMPs — became the best guess for
dark matter’s nature.
One initially appealing aspect of
WIMPs was that scientists thought they
knew how to detect the particles and
study their properties. Motivated by this
goal, physicists engaged in an ambitious
experimental program to identify these
WIMPs and learn how they were forged
in the Big Bang. Over the past couple of
decades, researchers have deployed a suc-
cession of increasingly sensitive dark
matter detectors in deep-underground
laboratories that are capable of detecting
individual collisions between a dark mat-
ter particle and the atoms that make up
the target.
These sophisticated experiments per-
formed beautifully — as well as or better
than designed. Yet no such collisions
have been observed. A decade ago, many
scientists were optimistic that these
experiments would bear fruit. But dark
matter has turned out to be very differ-
ent, and far more elusive, than we had
once imagined.
Although it’s still possible that dark
matter could consist of some form of
difficult-to-detect WIMPs, the lack of any
signal from underground experiments has
led many physicists to shift their focus
toward other dark matter candidates. One
such contender is a hypothetical ultralight
particle known as an axion. Axions are
predicted according to a theory proposed
by particle physicists Roberto Peccei andHelen Quinn in 1977. Although
scientists are searching for axions in
experiments that use powerful magnetic
fields to convert them into photons,
these investigations have yet to place
very strict constraints on the properties
of these particles.
Another possibility that could explain
why dark matter has been so difficult to
detect is that the first moments of the
universe may have played out much dif-
ferently than cosmologists have long
imagined. Take the case of the conven-
tional WIMP. Calculations show that the
f ledgling universe should have produced
vast quantities of these particles during
the first millionth of a second or so after
the Big Bang, when they reached a state
of equilibrium with the surrounding
plasma of quarks, gluons, and other sub-
atomic particles. The number of WIMPs
that could have survived these conditions
— and ultimately contributed to the dark
matter found throughout today’s uni-
verse — depends on how, and how often,
they interacted. But when carrying out
calculations such as these, scientists
generally assume that space expanded
steadily during the first fraction of a sec-
ond, without any unexpected events or
transitions. It is entirely plausible that
this simply was not the case.
Although cosmologists know a great
deal about how our universe expandedThe supercluster Abell 901/902 holds hundreds of
galaxies and massive amounts of dark matter. The
magenta-tinted clumps show the dark matter’s
distribution, derived from Hubble Space Telescope
observations, overlaid on a ground-based image of
the supercluster. HUBBLE DATA: NASA/ESA/C. HEYMANS (UNIVERSITY OF
BRITISH COLUMBIA) ET AL./THE STAGES COLLABORATION. GROUND-BASED IMAGE:
ESO/C. WOLF (OXFORD UNIVERSITY) ET AL./THE COMBO-17 COLLABORATIONThe blue light in this image of MACS J0416.1–2403
shows the arrangement of dark matter in this galaxy
cluster. Despite the ubiquity of dark matter in the
universe, astronomers still have no good idea what
it is made of. NASA/ESA/D. HARVEY (ÉCOLE POLYTECHNIQUE FÉDÉRALE DE
LAUSANNE)/R. MASSEY (DURHAM UNIVERSITY)/HST FRONTIER FIELDSThe Coma Cluster packs thousands of galaxies into a sphere measuring more than 20 million light-years
across. Fritz Zwicky discovered dark matter in this cluster in the 1930s when he deduced that the galaxies
are moving too fast to stay together unless the cluster contains nearly 10 times as much matter as what
can be seen. NASA/ESA/THE HUBBLE HERITAGE TEAM (STSCI/AURA)