Good question! Yes, before the early twentieth century, we thought of "space" not as an actual object, but just a bunch of nothing in which everything else existed. But with the development of special relativity in the early 1900s, we started to combine time and space into a single thing: spacetime. And with the advent of general relativity (Einstein's theory of gravity) in 1915, spacetime became dynamical: that means that we started thinking about spacetime as a real "thing" that can bend, stretch, squeeze, wave, and do all sorts of things. This bending of spacetime is what gravity is.

So spacetime (and therefore also space) really is a physical entity that can expand! What does this mean? Well, "space" is the thing through which we measure the distance between two objects, so if space is expanding, that really means that distances are expanding! This is hard to imagine: say I'm in a car and driving away from you towards my house. Then it's true that the distance between you and me is increasing, but is that because space is expanding? No - I'm moving away from you, but you're not moving relative to my house, and I'm moving closer to my house. In an expanding space, objects tend to ALL move farther apart - so I would be moving away from you, you'd be moving away from my house, and I'd be moving away from my house.

When we say our universe is expanding, this is what we mean: when we measure the velocities of galaxies relative to us, we see that everything is moving away from us, no matter what direction we look in.

Imagine you are a tiny organism living on the surface of a balloon. To you, space would be the surface of the balloon and you would measure the distance between two points on the surface of that balloon as the distance it took you to walk from one point to another. Now imagine that the balloon gets inflated so that it is larger. Now you would be required to walk a farther distance between the same two points than you would have had to walk before when the balloon was smaller. If you were a tiny organism living on the surface of this balloon, you would see that your universe has expanded. This is similar to what physicist mean when they say that our universe is expanding.

This is a tricky question to answer from far away. Space may be 'empty,' but it isn't actually nothing. It can be measured and has a size, despite being empty. Nothing is ...nothing. Nothing has no volume, and does not experience time. Many scientists say space (or spacetime) is like a fabric on which all things exist. I prefer to think of space is the surface of a balloon. To demonstrate this to yourself, blow up a balloon partially, and draw two galaxies on it, a few inches apart. Now, inflate the balloon more. The galaxies move apart, but have they really moved? They are situated on the same balloon surface places as before, and the drawings haven't moved, but there is more space in between them due to the balloon stretching out. This is the same way space expands. All we can see or interact with are the galaxies or the drawings, but the space underneath it all is expanding. Although the galaxies look like they are stretching a bit too, in reality, the universe is so big that it isn't noticeable to humans without very strong telescopes. Some estimates suggest that there are more stars than there are grains of sand on all the beaches in the world put together. The balloon example is also good, because many scientists believe that space is naturally curved so that if you go long enough in one direction you end up back at where you started, like on the balloon. To do this though, you would have to travel for billions of years at the speed of light. The universe is a big place.

If you're interested in questions like this, you may want to read Stephen Hawking's book A Brief History of Time. I read it in 11th grade, and it is written at a simple and interesting level. It is also actually pretty short (100 pages) and has sketches like the example I used above to explain these cool astronomy concepts. I'm sure he does a much better job than me :)

Simply put, space *is* a physical entity that *can* expand. It may be empty (i.e. devoid of matter), but it is not nothingness.

A way to visualize it is to think of a loaf of bread in an onion that has raisins in it; as the bread bakes, it expands, which causes the raisins to become farther apart. The farther apart the raisins are, the faster they move away from each-other as the loaf expands. Now, imagine that the bread is "empty" space, and that the raisins are galaxies. This is basically what the universe is doing. The only difference is that the bread has a limit to its dimensions, whereas space, as far as we know, extends to infinity in all directions. This means that the distance between two points in space is getting larger over time.

Space can contract, too. The presence of mass causes space around the masses to contract. This exerts a drag-like force on all other objects caught in the contracting space. This force is what we know of as gravity.