dwarf galaxy
If giant galaxies like the Milky Way are cities, dwarf galaxies are villages — small, faint, often overlooked, but vastly more numerous. A dwarf galaxy is simply a small galaxy, with anywhere from a few thousand to a few billion stars, far fewer than the hundreds of billions in a big spiral. They are the most common galaxies in the universe, even though their dimness makes them hard to spot.
Dwarfs come in several flavors echoing the big galaxies: dwarf ellipticals and the even fainter dwarf spheroidals (smooth, gas-poor, old), and dwarf irregulars (lumpy, gas-rich, still forming stars). Many orbit a larger galaxy as satellites — the Milky Way alone has dozens of known dwarf companions, including the Magellanic Clouds and a swarm of ultra-faint dwarfs. A striking feature is that the smallest dwarfs appear to be almost entirely dark matter: their stars move far faster than their meager starlight can explain, so a tiny pinch of stars sits inside a relatively large dark-matter halo.
Dwarf galaxies matter for two big reasons. First, in the hierarchical picture they are the building blocks: big galaxies grew by swallowing dwarfs, and dwarfs being torn apart leave the stellar streams we see threading galactic halos today. Second, because the tiniest dwarfs are so dark-matter-dominated, they are prized laboratories for testing what dark matter is — and certain mismatches between the number and density of dwarfs we observe and what simple simulations predict (the 'missing satellites' and 'core-cusp' puzzles) are active, honestly unresolved debates.
The Fornax dwarf spheroidal, a Milky Way satellite, contains only a few million stars, yet the speeds of those stars imply a total mass tens of times greater than the stars alone — strong evidence that it is dominated by an unseen dark-matter halo.
A handful of stars riding inside a heavy dark-matter halo — the universe's most common galaxy.
Dwarf galaxies are not failed or partly-formed galaxies; they are bona fide galaxies, just small. Globular clusters can hold similar numbers of stars but are not galaxies, because they lack the dark-matter halo and the spread of stellar ages that dwarfs have.