Cloud-9 is a galaxy with practically no stars in it - just gas and a large amount of dark matter! It seems to be a remnant of early galaxy formation: a 'failed galaxy' that never produced stars.
The core of this object, marked with the dashed circle here, is made of neutral hydrogen and is about 4,900 light-years in diameter. Researchers detected it by the radio waves it emits, and used these to estimate that the gas cloud is about one million times the mass of the Sun.
But assuming that Cloud-9 is held together against the pressure of the gas by gravity, a team estimated that its total mass must be more like five billion solar masses!
The rest of the mass is theorized to be dark matter. So: Cloud-9 may be a 'dark galaxy': a galaxy made almost entirely of dark matter. For more, read this:
There are other candidate dark galaxies, some much more massive:
https://en.wikipedia.org/wiki/Dark_galaxy
'Smith's cloud' is a fascinating small example. It's currently moving towards the disk of the Milky Way at about 75 kilometers per second - but extrapolating backwards, it seems to have gone through the Milky Way before!
https://en.wikipedia.org/wiki/Smith%27s_Cloud
People, especially those who aren't astrophysicists, like to cast scorn on dark matter. But any alternative theory for what's going on has to explain a lot of very phenomena, including these peculiar galaxies with almost no stars, that act much heavier than the gas in them can explain. I'm hoping that dark matter will someday be understood, and that this will lead to a great breakthrough in particle physics. For now, patient astronomers keep collecting evidence.
