How can it have mass / interact gravitationally and not make "clumps"?

Gravity only pulls things together, it doesn't stop them when they get close to each other. In the absence of "friction" (all other interactions except gravity), things get close to each other and just zoom past by / through each other.

If you only have gravity but nothing else, there are no collisions.

If you've seen the CUDA demo where they simulate thousands of bodies interacting by gravity, and they just keep swarming around forever - it's like that. No friction or stickiness, no collisions, just forever swarming around.

Dark matter, whatever it really is, is rather like a "ghost". Just goes through normal matter and through itself and doesn't do anything except a little gravitational tug.

See http://physics.stackexchange.com/questions/214950/if-dark-ma...

TL;DR: Dark Matter doesn't clump because of missing "friction" from the other forces, esp. electromagnetic/weak.

upvoted, and not disagreeing (b/c i know nothing a/b this) -- would like a similarly semisimple explain on how "interaction" thru gravity alone makes any sense at all in the relativity framework: light no longer travels along lines of zero acceleration?

Edit: on second thought, a book reference is probably better

In relativity, gravity isn't a force that causes acceleration, it's a change in the shape of space which changes the path objects take. Thus, gravity affects the path light takes, but that's still a "straight line," because what gravity is really affecting is what a straight line is.

This is my favorite explanation of this, mind if I forward this to an old Astronomy Professor? I think he'd love this too.

Sure! Just beware that I'm far from an expert and may well have gotten something hilariously wrong. If he rips it to shreds, please let me know what he had to say.