Okay, I know this is cheesy, but check out the OST to the movie Dune. It has some good "epic" sounding tracks.
Star Trek Wrath of Khan OST - Battle of Mutara Nebula. Cheese, but fine cheese. James Horner is the unsung hero of movie soundtracks.
For a more exotic flavour in the space battles, Kodo demon drummers.
You might be able to tease out something useful from the modern composers (if you can stomach them - its really a matter of taste). I'm thinking Phillip Glass, Steve Reich. But you'll have to cherry pick. Good for grand modern vistas like entering bustling starports, etc.
For more electronic technological hustle and bustle, try Squarepusher, Ultravisitor. All instumental, often cacophonous, but possibly useful.
Depending on what you need, you might want to try The Clock from Thom Yorke's The Eraser. Very intense track. Useful for anything heavy.
Speaking of erasers, try the track Eraser from NIN's the Downward Spiral for anything requiring buildup... or maybe not, all that screaming at the end will be distracting
These suggestions may be total bunk of course depending on your taste in music and the mood and flavour you're looking for in your campaign.