Within the BP!Verse, the Anti-Void is considered to be everything in one place. There’s just one problem. Everything is a lot of things, so the way the Anti-avoid works is a constant movement of these things in a constant loop. Occasionally, something will slip out, but that something is usually something small like a penny or toothbrush. That’s why sometimes you might come across a random sock on the floor.
The nature of the Anti-Void also means it has sentience. It harbors certain opinions of people who enter it, and even people outside of it’s bounds, because they technically exist there even if they don’t. Sans (AKA BP!Error) is unlucky, for the Anti-Void doesn’t really like him. On the other hand, it seems to really like Pink(BP!Ink), so them being friends kinda evens things out. Thanks to some of Sans’s abilities, he’s the only one who can see what the Anti-Void is saying. Sadly, since he lives within the Anti-Void, before Pink found Sans, the Anti-Void and Sans were far from friends (not that they are now).