10 ASTRONOMY • APRIL 2020
QUANTUM GRAVITY
Astronomers have uncovered
19 small galaxies that appear to
be deficient in dark matter. The new
finding, published November 26 in
Nature Astronomy, bolsters a recent,
controversial discovery of two other
galaxies apparently devoid of the
mysterious substance.
Dark matter, which accounts for
some 85 percent of the matter in the
universe, is thought to be the primary
component of all galaxies — as well as
the main driver of galaxy formation
in the first place. So, finding so many
galaxies apparently lacking the exoticmatter indicates that astronomers might
be missing something major about how
some galaxies form and evolve.
“This result is very hard to explain
using the standard galaxy formation
model,” said lead author Qi Guo of the
Chinese Academy of Sciences in a press
release, “and thus encourages people to
revisit the nature of dark matter.”
The latest batch of galaxies missing
their dark matter was discovered when
Guo and her team explored the nature
of 324 dwarf galaxies. Using data from
the Arecibo Observatory in Puerto
Rico and the Sloan Digital Sky Survey,they analyzed how fast hydrogen gas
rotates around each galaxy. They also
calculated how much normal matter —
in the form of both gas and stars — the
galaxies contain.
After crunching the numbers, Guo
and her colleagues determined that 19
of the hundreds of dwarf galaxies they
investigated contain enough visible
matter to explain the motions of the
galaxies’ hydrogen. In other words,
these galaxies seem to be missing the
expected amount of dark matter.
According to the study, the “results
suggest that a population of dwarfMORE GALAXIES FOUND TO
BE MISSING DARK MATTER
New results may have dramatic implications for
theories about both dark matter and galaxy formation.
SMALL PACKAGE. Dwarf galaxy NGC 5477
is like most galaxies: It has much more dark
matter than regular matter. But the recent
discovery of 19 dwarf galaxies missing most
of their dark matter may force astronomers to
reconsider how such galaxies form in the first
place. HUBBLE/ESA/NASA