Anti-matter is a type of thing.

As humans can't see anti-matter, we don't really know what sort of stuff it's made from, if it is made of any sort of stuff at all. There's no atoms or molecules or anything like that in it (or if there are, they are not like the atoms and molecules that we can see).

It's probably made of waves or subatomic particles that we don't know about yet, or something.

See also:

Dark matter