Mryntu

Notes:
1. I'm in 8th grade so i don't have in-depth schooling on this. Lengthier explanations with more extra information that would improve understanding would be much appreciated.
2. I asked a question on this topic earlier so this is an expansion of it.
3. English isn't perfect yet, so if anyone can give either Russian, Arabic, or Mandarin translations of complex terms, this would also help very much.

Many of the responses mentioned the big bang, and I thought that maybe it's continuous expansion, if the expansion is throughout and not new mass added on the edge of expansion, is this what balances out gravity? And if so, can its rate of movement (due to the loss of energy over time) decrease to a point that gravity overpowers it, and all mass would collapse in on itself?

The mere fact that the universe is expanding doesn't mean that it can't collapse, any more than a stone moving upward means it can't turn around and come back down. The universe eventually would stop expanding and collapse back to a Big Crunch if it had enough matter and no dark energy.

But there apparently is dark energy, and it is making the universe expand faster and faster, not slower and slower! The anti-gravity of dark energy has been the dominant force in the universe for the last five or six billion years. Matter is no longer particularly important. According to the standard Lambda-CDM model of cosmology, dark energy now constitutes 69.11% percent of the energy density of the universe, and matter (both visible and dark) only 30.89%.

According to the model, the universe will continue to expand forever, faster and faster, with the Friedmann scale factor $a(t)$ that determines its size eventually doubling every 11.6 billion years. The fraction of the energy density due to matter will approach 0%, while the fraction of the energy density due to dark energy will approach 100%. There is no Big Crunch in our future, according to the model.

Note: There is no "edge" to the expanding universe. Galaxies are not expanding into empty space "beyond the edge". Instead, space is expanding everywhere between galaxies, and there is no distance far from us at which you stop finding galaxies. Also, there is no new mass being added.

If there were no dark energy, the universe would be expanding slower and slower rather than faster and faster. If there was enough matter, it would turn around and collapse. If there was not enough matter, it would keep expanding. It would be like a rocket launched from Earth. If a rocket isn't launched with escape velocity, it falls back to Earth. If it has exactly escape velocity, it can reach infinity with zero velocity. If it has more than escape velocity, it can reach infinity with positive velocity. This is similar to how the universe would work if there were no dark energy.

I will try to give a simple example:

You know that all mass attracts all other mass, but in such a weak way, that only with large bodies can strong enough gravitational fields exist, as with the earth, which can hold the moon in an orbit around it with their mutual attraction.

When throwing a ball , your muscles supply kinetic energy, and it moves, but the earths gravitational pull makes the track a parabola. In space far from the earth, a thrown ball would go straight. The gravitational pull between small masses of each other is very weak , because of the small numbers in front of the formula:

$F=Gm_1m_2/r^2$

Where F is the force, m1 and m2 are the masses of the objects interacting, r is the distance between the centers of the masses and G is the gravitational constant, a very small number.

Because the force for small masses is very weak , when the masses are moving, i.e. have kinetic energy, the effect of the gravitational pull on the trajectory of the small masses can be ignored.

On the surface of the earth we just have a constant g , and only use the attraction of masses to the huge mass of earth.

Take a bomb and trigger it to explode. Why does not the gravity of the individual pieces exploding pull back together? Because chemical energy was supplied and turned into kinetic energy and the pieces flying away have a very small attraction to each other due to their gravity, so small, that the tracks of the break up are straight lines until the earth's large gravity gets them.

The original Big Bang model is about elementary particles and radiation exploding ( energy provided not known) into space. The model , after the energy is dissipated could end up into falling back into itself, in what is the Big Crunch model.

So what is keeping the universe at our time stable is the kinetic energy that masses acquired at the Big Bang, with trajectories which are affected by each other making galaxies etc, but the over all effect still is that clusters of galaxies are moving away from each other , not converging, in a continuous explosive like expansion of space.

Since you are interested in the subject you could read the links I have provided.

For an accurate explanation you should try to understand the answer by G.Smith.