eli5 why we never hear about supermassive objects which aren’t hot/bright

Question

For example, some giant “planet” the size of our sun which has a surface like the moon, floating through space in darkness.

Tangentially, how are we sure that black holes aren’t these? Are we misinterpreting absence of light as black holes when instead they could just be large dark objects?

In: 9

Answer 1

>supermassive objects which aren’t hot/bright

I don’t know, we have a giant black hole at the center of the galaxy, I wouldn’t say we never hear about it.

>Tangentially, how are we sure that black holes aren’t these? Are we misinterpreting absence of light as black holes when instead they could just be large dark objects?

We don’t detect black holes by looking for absence of light in them. We know black holes are where they are because of the gravitational well they produce, as well as the lensing effect light (and all electromagnetic radiation) go through when traveling next to the black hole.