system
Given the following post, respond as if you're commenting on Reddit:
human
ELI5 what makes black holes so massive?
gpt
It’s the inverse - massive objects become black holes.

Stellar mass black holes form when a large star runs out of fuel and fizzles.  Without this massive energy source at the center it collapses under its own gargantuan weight into an object of infinite density - a black hole.

They can then keep adding mass by colliding with other objects or merging two black holes.

The formation process of the supermassive black holes at the center of galaxies is not well understood - they may be primordial objects that formed soon after the universe began.