This idea, came to me via a very weird dream. but here it is, pretty sure i saw similar ideas about by others though

Was Gaster the father, a brother or creator.

He was the brother.

The only big brother.

Then who was Sans?

When Gaster fell into the core, the void shattered him into two.

Void Gaster we all know as that goopy figure and Sans, Sans who was greatly weakened by this action and his HP frozen at 1.

He left to live at snowdin as no one remembered him but Papyrus due to there brotherly bond, as he no longer looked the same and he was kind out of a job due to this.

The problem is, due to the reset Sans is getting weaker and the void is ever hungry to claim the remains left behind.