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

Simply put, once objects get a certain size, they pretty much only can exist as a star, black hole, etc. A solid mass the size of the sun could never exist because it would collapse under its own gravity. And once an even larger mass is achieved, it collapses under its own gravity to become a black hole. And we find black holes because we see objects like starts orbiting a point of seemingly nothing at breakneck speeds.