Theoretically speaking what would a galaxy "look" like or its characteristics be if all stars in it have collapsed into black hole. Would it be brightest galaxy ever or black?
Edit: Followup. If black then how would one be detected?
Probably extremely dark. The only black holes that are "bright" are so because they have accretion disks of concentrated gas and dust that heats up before it falls into the event horizon. A galaxy that has only black holes and no more stars would also be extremely poor in gas and dust, otherwise new stars would be forming. And other sources of accretion material, like tidally disrupted stars, would also be missing. Presumably there would still be some neutron stars and white dwarfs around, so now and then there could be tidal disruption and accretion events that light up some black holes (generally the central supermassive one), but it would be a very irregular event.
Similar to dark matter, which already outweighs visible matter in a 5:1 ratio. Impossible to see directly, but has an observable gravitational effect on other objects.
Probably extremely dark. The only black holes that are "bright" are so because they have accretion disks of concentrated gas and dust that heats up before it falls into the event horizon. A galaxy that has only black holes and no more stars would also be extremely poor in gas and dust, otherwise new stars would be forming. And other sources of accretion material, like tidally disrupted stars, would also be missing. Presumably there would still be some neutron stars and white dwarfs around, so now and then there could be tidal disruption and accretion events that light up some black holes (generally the central supermassive one), but it would be a very irregular event.
It's called a ghost Galaxy and they're theoretical. You wouldn't be able to see them
One way we would be able to detect them is via gravitational lensing of the light of visible galaxies behind them.
Similar to dark matter, which already outweighs visible matter in a 5:1 ratio. Impossible to see directly, but has an observable gravitational effect on other objects.