What's the Name of That Book??? discussion

Hi guys!
So the title pretty much sums it up, I'm looking for a book about an astronaut from Earth that goes into space and finds a galaxy made of dark matter/anti matter where he meets a girl made of antimatter.
I don't know when this was published, I guess it goes in the sci fi genre and I don't really know anything else about the book seeing as I only heard this and have no way of asking the person I heard it from.
So thank you if anyone knows the title xoxo

Does it involve romance with the girl made of antimatter?

Lex, what year did you hear about this book?

Lex, are you still looking for this or did you find it?

No response, moving to Abandoned.