www.universeadventure.org:

"Planets, stars, and galaxies are bound together by gravity. On the short distance scales present in these systems the force of gravity is great enough to resist and stop the expansion of space. Gravity opposes the expansion of the universe on larger scales as well, but since the strength of gravity falls with the square of the distance, the force is not enough to halt the expansion. If every object expanded with space, including ourselves, we would not perceive any expansion at all."

And at even smaller scales the strong nuclear force and the electromagnetic force overcome the expansion of space between particles.

www.particleadventure.org:

"What holds it together? The universe, which we know and love, exists because the fundamental particles interact. These interactions include attractive and repulsive forces, decay, and annihilation.
There are four fundamental interactions between particles, and all forces in the world can be attributed to these four interactions!
Gravity; Electromagnetic; Strong; Weak.
That's right: Any force you can think of -- friction, magnetism, gravity, nuclear decay, and so on -- is caused by one of these four fundamental interactions."

"The electromagnetic force causes like-charged things to repel and oppositely-charged things to attract. Many everyday forces, such as friction, and even magnetism, are caused by the electromagnetic, or E-M force. For instance, the force that keeps you from falling through the floor is the electromagnetic force which causes the atoms making up the matter in your feet and the floor to resist being displaced."

"Atoms usually have the same numbers of protons and electrons. They are electrically neutral, therefore, because the positive protons cancel out the negative electrons. Since they are neutral, what causes them to stick together to form stable molecules?
The answer is a bit strange: we've discovered that the charged parts of one atom can interact with the charged parts of another atom. This allows different atoms to bind together, an effect called the residual electromagnetic force.
So the electromagnetic force is what allows atoms to bond and form molecules, allowing the world to stay together and create the matter you interact with all of the time."

"Quarks have electromagnetic charge, and they also have an altogether different kind of charge called color charge. The force between color-charged particles is very strong, so this force is "creatively" called The Strong Force.
The strong force holds quarks together to form hadrons. And while quarks have color charge, composite particles made out of quarks have no net color charge (they are color neutral). For this reason, the strong force only takes place on the really small level of quark interactions, which is why you are not aware of the strong force in your everyday life."

"So now we know that the strong force binds quarks together because quarks have color charge. But that still does not explain what holds the nucleus together, since positive protons repel each other with electromagnetic force, and protons and neutrons are color-neutral.
So what holds the nucleus together? Huh?
The answer is that, in short, they don't call it the strong force for nothing. The strong force between the quarks in one proton and the quarks in another proton is strong enough to overwhelm the repulsive electromagnetic force.
This is called the residual strong interaction, and it is what "glues" the nucleus together."



Hope this helps?