Scientific American - USA (2022-04)

Graphic by Matthew Twombly April 2022, ScientificAmerican.com 61

tific projects to a congressionally mandated panel that
will determine scientific priorities. For the first time, cos-
mic probes of dark matter will be a distinct topic of con-
sideration. Although Snowmass does not make formal
policy recommendations, it is certainly the case that at
each stage of the organizational hierarchy, there will be
decisions about what science to emphasize.

A UNIVERSE OF DARK MATTER CANDIDATES

there is still muCh we don’t know about dark matter,

but we have come a long way since Rubin’s work in the

1970s and 1980s. We now have good evidence to suggest

that every galaxy lives in its own bubble of dark matter—

called a dark matter halo—that extends well beyond the

visible part of the galaxy. The amount of dark matter in

The Dark Matter Hunt

Scientists know the matter we can see is not all that is there.

But so far attempts to find dark matter and discover its

identity have not been successful. The search continues.

Centers of Galaxies

If dark matter is its own

antiparticle—a possibility

under the weakly inter­

acting massive particle

(WIMP) model—it would

annihilate itself at the

centers of galaxies to create

an excess of gamma rays

that telescopes could see.

Galaxy Clusters

The same phenomenon—

WIMP particles destroying

themselves—could be

evident in clusters of

galaxies, where dark

matter is also expected

to be concentrated.

The CMB

The cosmic microwave

background (CMB) is

ancient light that pervades

the universe. Patterns in the

frequency of this light reflect

how much total mass was

present in the early cosmos,

constraining different dark

matter candidates.

Gravitational Lensing

Scientists can look at

how mass bends light as

it travels through the

universe—a phenomenon

known as gravitational

lensing—to study how

much mass dark matter

contributes and what

it might be made of.

Neutron Stars

These dense spinning stars

may produce axions—

one dark matter candidate—

inside their cores. If so,

the axions could decay into

photons telescopes could

see. They would also cause

neutron stars to lose heat in

a measurable way.

FOUR WAYS TO LOOK

FOR DARK MATTER

Direct detection experiments, such as the

Xenon1T detector in Italy, look for signs of

dark matter particles interacting via the weak

force with regular matter. Particle colliders,

such as the Large Hadron Collider near Geneva,

search for dark matter particles among the

debris created when normal particles smash

together. Indirect detection experiments,

including astrophysical searches such as the

High Energy Stereo scopic System (HESS)

in Namibia, look for signs of dark matter

interacting with itself, producing normal

matter in the process. Indirect detections

often involve looking for the aftermath

of cosmic particles interacting with the

atmosphere. All of these methods require

dark matter to interact with traditional

particles in some way besides gravity.

If it does not, observations of astro physical

objects may be the only way to find the

hidden material.

ASTROPHYSICAL SIGNALS

DIRECT

DETECTION

EXPERIMENTS

PARTICLE COLLIDERS

INDIRECT

DETECTION

EXPERIMENTS