Why don't stars clump together?

Star systems are too far away from each other to be significantly affected by each other’s gravity.

On top of what @chocolatechip said, maybe they will, eventually. But since they’re so far apart, it takes them billions of years.
They also still seem to have momentum from the Big Bang, propelling them away from each other.

Well, if the marbles in your example were spaced out too far away, then the attraction between them would be quite weak to produce any “quick” results like you explained.

The distances between stars (even within the same galaxy) is similarly quite vast compared to the sizes of these stars.

The stars do attract each other. The strength of this attraction depends on the distances separating them. Also, how soon, if at all, they will fall back into each other owing to this mutual attraction will be determined by…
1. this distance between stars
2. their current velocities (which includes not only their speed but also the direction in which they are headed. You can appreciate that if two magnetic marbles are thrown away from each other to start with, they will take a long time to come back together, or may not even come back together at all.)

Another important factor is the time scale. The whole of Human existence (estimated to be approx. 250,000 years now) is a mere blink in the universal time scale (universe is estimated to be more than 13 billion years in age)

So within a single human life span (~ 100 years) we will hardly notice any dramatic changes – even if the stars were indeed collapsing into each other. We would only be able to make measurements of their positions and velocities and deduce that they are either collapsing or receding from each other.

(This is similar to our inability to see mountains forming, or notice the river banks eroding—these phenomenon take thousands of years to produce any noticeable change and hence are not perceivable with the senses that we individuals have.)

Current scientific knowledge seems to indicate that the stars (and galaxies) are in fact moving away from each other – possibly the continuation of what started as the “big bang”.

Hope this helps.

Well, actually they already do, albeit on a very large scale:

http://www.wikinfo.org/upload/2/2e/Andromeda_Galaxy_(small).jpg

Stars clumping together is what makes galaxies.

Stars also clump on a small scale – from that page: “It is estimated that approximately 1/3 of the star systems in the Milky Way are binary or multiple, with the remaining 2/3 consisting of single stars.”

@davidgro I didn’t know it was as many as ⅓. Thanks.