WWW.ASTRONOMY.COM 45
ome 60 million light-
years from Earth —
by some researchers’
estimate, anyway — a
pair of strange galax-
ies is causing a cosmic
stir. These island uni-
verses hold far fewerstars than your average galaxy. But it’s
not the lack of stars that surprises astron-
omers. The bizarre galaxies, named NGC
1052-DF2 and NGC 1052-DF4 (or DF2
and DF4, for short), also seem to lack any
significant amount of dark matter.
Because the duo could be the first
known galaxies without the substance,
which accounts for 85 percent of the uni-
verse’s matter, news of DF2’s discovery in
2018 quickly spread throughout the
astronomical community. If confirmed,
such galaxies without dark matter would
throw a wrench into our understanding
of how galaxies form and evolve.
“We thought that every galaxy had
dark matter and that dark matter is how
a galaxy begins,” astrophysicist Pieter van
Dokkum of Yale University, lead author
of the initial paper on DF2, said in a
press release. “This invisible, mysterious
substance is the most dominant aspect of
any galaxy. So finding a galaxy without it
is unexpected. It challenges the standard
ideas of how we think galaxies work, and
it shows that dark matter is real: It has its
own separate existence apart from other
components of galaxies.”
But as Carl Sagan liked to say,
extraordinary claims require extraordi-
nary evidence. And, according to some
researchers, the evidence for these dark-
matter-deficient galaxies doesn’t hold up.
What is DF2?
DF2, the first known galaxy supposedly
devoid of dark matter, is a member of
a unique class of galaxies called ultra-
diffuse galaxies (UDGs). Although
UDGs can grow as large as the Milky
Way, these hazy specters contain
hundreds to thousands of times fewer
stars. And because they’re essentially
transparent, they’re extremely difficult
for astronomers to observe in detail.
The relatively innocuous DF2 first
stood out to researchers when they iden-
tified it using the Dragonf ly Telephoto
Array. This robotic collection of Canon
400mm telephoto lenses combine toStationed in New Mexico, the novel
Dragonfly Telephoto Array has been built
up over the years into a collection of 48
telephoto lenses bundled in two groups.
Together, the 400mm lenses create the
equivalent of a 1-meter refracting telescope
and enable observations of extremely faint
and diffuse cosmic structures. P. VAN DOKKUM (YALE)Astronomers uncovered two see-
through galaxies that appear to be
devoid of dark matter. And because
they lack the strange substance, their
globular clusters move much more
slowly than expected. ASTRONOMY: ROEN KELLY