Dark energy makes up some 74% of the Universe...

[Volcano1985]

Dark energy makes up some 74% of the Universe and its existence would explain why the Universe appears to be expanding at an accelerating rate.

Why would? Because

- Used to express presumption or expectation?

[Rover_KE]

I would say that's a reasonable assumption.

Rover

[5jj]

I would agree with that.

[Raymott]

Dark energy makes up some 74% of the Universe and its existence would explain why the Universe appears to be expanding at an accelerating rate.

Why would? Because

- Used to express presumption or expectation?

It sounds like a strange sentence to me. If they know dark matter exists, then why not "It does explain...?"
If they weren't certain it existed, "would" would be correct, but they assert that it does exist. What is the expectation then?

Perhaps their doubt is whether the universe is expanding at an accelerating rate. In fact, they only claim that it appears to be. If that's why they used 'would', they mean that it would explain the expansion, not just the appearance of expansion. The actual expansion (not the existence of dark matter) explains the appearance of expansion.

I think they mean that dark matter would explain the expansion, if indeed expansion is happening.

[Volcano1985]

Why not?

... its existence may/might/could explain why the Universe appears to be expanding at an accelerating rate.

[Raymott]

Why not?

... its existence may/might/could explain why the Universe appears to be expanding at an accelerating rate.

Indeed, why not? I think any of those is more logical.