How do atheists explain the universe or multiverse?

The first law of thermodynamics states that energy can be transformed, but cannot be created or destroyed. If it cannot be created, the mass of our universe should be zero, which it is not. Why?

If our universe is part of a multiverse, how do atheists explain the natural laws of the multiverse (I call them meta laws to distinguish them from the natural laws we observe) which include a universe-generating mechanism?

I would welcome comments which include more than just “well, that’s just the way it is”. Thanks in advance.

A second related question is:

What do believers and agnostics think about the explanations for the universe given by atheists?

So atheists first, please.