60 Scientific American, April 2022
If I disappointed her with my answer, she didn’t
show it. Instead she asked me to sit down to lunch with
her and some other women astronomers, including for-
mer nasa administrator Nancy Grace Roman. Rubin
then proceeded to fangirl over Roman, who is often
referred to as “the mother of the Hubble Space Tele-
scope.” It was quite a moment for me, to watch an
elderly woman who had uncovered one of the greatest
scientific mysteries of our time excitedly introduce us
to her own hero.
Rubin cemented her legacy in the 1960s, when she
studied stars inside galaxies and found something odd:
stars on the outskirts of galaxies were moving faster
than they were supposed to, as if there was an invisi-
ble matter there contributing a gravitational pull. Her
work echoed findings from galaxy cluster studies in the
early 1930s by Fritz Zwicky, which had led him to sug-
gest the existence of Dunkle Materie, German for “dark
matter.” Throughout the 1970s Rubin and astronomer
Kent Ford published data consistent with this conclu-
sion, and by the early 1980s scientists were in wide-
spread agreement that physics had a dark mat-
ter problem.
Most attempts to track down dark matter in the lab-
oratory have fallen into three categories. So-called
direct detection experiments look for evidence of dark
matter particles interacting with particles of normal
matter—for instance, the element xenon—through one
of the nongravitational fundamental forces, the weak
force, as well as through hypothesized new forces. Col-
lider experiments, such as those at the Large Hadron
Collider near Geneva, take the opposite approach,
smashing two regular particles together with the hope
of producing dark matter particles. Meanwhile “indi-
rect detection” experiments look for evidence of dark
matter interacting with itself, with the resulting colli-
sion producing observable particles.
So far none of these strategies has turned up the
missing matter. We still don’t know if dark matter can
talk to regular matter in any way beyond gravity. It may
be impossible to produce in the accelerators we can
build or to detect in the experiments we can construct.
For this reason, astronomical observations—cosmic
probes of dark matter—are one of our best hopes. These
probes allow us to look for signatures of dark matter in
environments that are difficult for us to produce on
Earth—for example, inside neutron stars. More broadly,
such searches look at dark matter’s behavior under
gravity in a variety of locations.
Despite the promise of this approach for studying
dark matter, it has sometimes been caught in the mid-
dle between the astronomy and physics communities.
Physicists tend to emphasize colliders and laboratory
experiments and don’t always prioritize links to astro-
physical work. Astronomers tend to write dark matter
off as a particle physics problem. This disconnect has
implications for funding. In 2022 we have an opportu-
nity to change that. The start of the 2020s marked the
beginning of an important process known as the Snow-
mass Particle Physics Community Planning Exercise.
This project, which takes place about once a decade,
brings physicists together to explain prospective scien-
H
ow do you think the dark matter problem is solved?” vera C. rubin urgently
asked me, within minutes of being introduced at a 2009 Women in Astronomy
conference. To this day, I can’t remember what I said in response. I was awe-
struck: the famed astronomer who had won the National Medal of Science for
her work finding the first conclusive evidence for dark matter’s existence was
asking me, a twentysomething Ph.D. student, for my opinion. I am certain that
whatever I came up with was not very good because it was a problem that I had,
until that moment, given no serious thought to. Until Rubin asked me my opinion, it had never
occurred to me that I was entitled to have an opinion on the question at all.
Chanda Prescod-Weinstein is a theoretical physicist. She is
an assistant professor of physics and a core faculty member in
women’s and gender studies at the University of New Hampshire.
She is author of The Disordered Cosmos: A Journey into Dark
Matter, Spacetime, and Dreams Deferred (Bold Type Books, 2021).
