I think everything in the Universe is useful for some purpose. Can you name anything that is completely useless?

Apparently the word "useless" itself, from what you're saying

If word "useless" is actually useless then the word "useless" cannot be useless

Oops, Russell's paradox strikes again

Most of the universe is completely useless. And I mean unbelievably vast quantities of space and matter.

Space and time can be quantum error correcting code. Also the Universe might not be stable for us to live if the matter and space is missing.

https://www.quantamagazine.org/how-space-and-time-could-be-a...

>Also the Universe might not be stable for us to live if the matter and space is missing

Let's put it this way: a million galaxies could blow up entirely to bits, and as far as we are concerned it wouldn't even make any difference.

It's probably useless for us now but a space faring civilization can use them for expanding their civilization :)