Wow, that is a good question that scientists are struggling to figure out! The answer is, we don't know how! We know that it is expanding, because we can measure the red shifts of galaxies and distant supernovas, and we can tell that the universe is expanding. But how and why? That is the subject for huge debates right now.

To begin to answer your question, I need to tell you about what we believe the universe is made of:
At the moment, our best measurements tell us that only about 5% of the stuff in the universe is normal stuff like us, and like stars; about 25% of the stuff is in some dark form that we can "feel" because it has gravity, but we can't see it because it does not radiate. But we don't know what that dark matter is, because it does not seem to be localized in the form of planets and other bodies. Rather, it seems to be collected all around and in between galaxies, and is in the form of some dark matter "particles" that we have not yet been able to detect. For all we know, these dark matter particles are streaming through the earth, and our bodies, right now! But, that is not the stuff that would make the universe expand, because it has gravity.

The strangest stuff is something that we call "dark energy" for lack of a better term. That is the strangest stuff of all, because it might be something like "anti-gravity." That dark energy, we think, is what is causing the universe to expand. Even more strange is that it seems to keep "making itself." It has a constant density, no matter how much the space between galaxies expands.

Here's the difference between a constant amount of gas in an expanding container, so that the density of the gas (number of molecules per cubic centimeter) goes down, and this dark energy, which keeps constant density as the universe expands - which means there has to be MORE of it being created all the time.

Say you took just an ordinary balloon and tied it shut. It has a little bit of air in it, right? Say you put that tied off, pathetic little balloon into a vacuum chamber, and pump out all the air from around the balloon. That little balloon would "blow itself up" as the air inside it, which was at atmospheric pressure when you had it just in the room, causes the balloon to expand when you remove the air pressure from around it. Have you seen your teacher do this? You should ask her or him to do this for you. It's pretty cool...you can put a marshmallow into a vacuum chamber and it also blows up and looks pretty cool.

Anyway, when that little balloon with the small amount of air in it gets all the air pressure removed from outside it, the air inside expands. But, there is NO MORE air inside than when you first tied it off - the air molecules are just freer to move around now. And since there is a larger space (volume) for them to move around in, the density of air inside the balloon goes down. (Because density = mass/volume, so if the volume gets bigger, the ratio gets smaller.)

Ok, well this dark energy is NOT like the air in that balloon! As the universe expands, this dark energy just keeps popping into existence, so that its DENSITY is CONSTANT. That means there is always more of it. But it has a weird property of NEGATIVE pressure - it behaves just the opposite of gravity. It makes the universe expand, and the more of it there is in the universe, the faster the universe expands.

So, at the moment, the universe is expanding faster than it was 5 billion years ago, when our solar system first formed. And in another 5 billion years we think it will be expanding even faster.

Don't worry, however... this all is way too far into the future to worry about. fun stuff to think about, huh?

One simple way to understand this is to imagine the universe to be two-dimensional, instead of three-dimensional, and space to be the surface of an inflated rubber balloon. Imagine tiny dots on the surface to be galaxies. As you inflate the balloon, the distance between galaxies increases, even though the size of each galaxy remains the same. There is a balance between how fast galaxies are moving apart from each other at one particular time, on one hand, and how much they attract each other by way of their mutual gravitational pull, on the other hand. If the latter wins, the expansion will eventually stop and the universe will begin to contract. Otherwise, the expansion will continue forever.

A thing to note is that no mass or energy is being added to the universe. It simply increases in size, not in content. In other words, as the universe expands, more space is being created continuously.

There are two answers: the simple one and the complicated one. The complicated one is more correct, but requires the simple one to understand the magnitude of.

The simple one is momentum. The universe is already expanding, and it takes time and force to cause the expanding galaxies to slow their expansion. It's just as if you throw a ball up into the air: it takes time for gravity to bring it to a stop so that it can comeback down again, and if you threw it hard enough (which you can't as a human), the ball would leave the Earth and never actually stop.

The truth, however, is far more complicated. The universe is accelerating its expansion (just as a rocket leaving the Earth continues accelerating from its engines). There seems to be some strange, repulsive effect of gravity at a large scale, perhaps involving undescribed forms of energy or matter. We do not understand it. Nonetheless, this is what we observe.

I am not exactly sure how the universe keeps getting bigger. It is not an easy thing to think about space as we know it expanding into even more space. To be honest, thinking about this gives me a lot of headaches! However, the reason that scientists are believing that the universe is expanding is due to the Doppler Effect. Have you ever heard a motorcycle pass by? As its buzzing sound approaches it seems to get louder and then as it passes it just 'zooms' by in a different type of tone. This an example of the doppler effect -- The change in pitch is a result from a shift in the frequency of sound waves.

In astronomy they look at the Doppler effect using light waves. Astronomists first used the Doppler effect to find how fast stars or other object moved toward or away from the earth. To do this they looked at shifts in spectrum of gases from the star compared to the same gases on earth. For the universe they believe it is expanding by looking at the spectral shifts caused by the Doppler effect of the edge of material blown up by the Big Bang.