Astronomers have long known that small dark-matter dominated galaxies exist. None were supposed to be as big as ordinary spiral galaxies such as NGC 3810, seen here in negative. Photo illustration by Olena Shmahalo/Quanta Magazine; Source: ESA/Hubble and NASA
Astronomers have long known
of small dark-matter dominated
 galaxies. None were supposed
to be as big as ordinary spiral
galaxies such as NGC 3810,
seen here in negative.
Among the thousand-plus
galaxies in the Coma cluster, a
massive clump of matter
 some 300 million
light-years away, is at
least one — and maybe
a few hundred — that
shouldn’t exist.