Rev, let's try asking and answering the question in both of those forms.

1. If the universe is all there is, how would you know it was rotating?
You wouldn't.

2. If the universe is all there is, why would you know it was rotating?
Since you couldn't know, he question seems to have no relevance.

Perhaps we could say: "Why would you not know". Then the answer would be: "because you would perceive everything as being stationary relative to everything else".

Could be you had something different in mine, though.

There never was nothing.