Question

Why does the size of the universe change?

Answer 1

Expansion

Explanation:

There are many theories on why the universe is expanding, however one of the more common ones is that after the Big Bang, and explosion from the central point, the particles are still all moving away in all different directions, and so we continue to move 'out'.

Answer 2

See explanation.

Explanation:

I confine myself to change only.

What matters most in our universe is matter.

In a specific type of matter, dimensional changes occur due to

different natural phenomena.

Broadly, they are absorption/expansion, contraction/disintegration,

and expansion/integration.

Accordingly, the space occupied changes and the relative space in-

between changes.