Matter is a lot like energy.
It can't be created or destroyed. It can only be moved around, shifted, rearranged, transformed, disguised, etc. Never created or destroyed.
If you suddenly see some matter that wasn't there a moment ago, it had to come from somewhere. It couldn't just appear out of nothing.
If you suddenly STOP seeing some matter that was there a moment ago, it had to go somewhere. It couldn't just disappear into nothing.