When I think of the Universe, I think of space. Lots and lots of space. There are around 100 billion galaxies, although I have heard that there might be more, and most galaxies contain at least 100 billion stars. Our Milky Way galaxy alone has between 100 and 400 billion stars and is a hundred thousand light years across. Our galaxy is so big that its gravitational attraction is strong enough to have smaller galaxies orbit it the way the moon orbits the earth.
But what the universe is made of is energy. Enormous amounts of it. Even empty space has energy. Most of the mass of elementary particles comes from the huge amount of energy inside those particles. Energy created the universe and energy causes it to expand at an ever accelerating rate. Energy created space itself.
Empty space is not empty, but full of fields that have energy in them. Everywhere there are fields that correspond to each elementary particle that makes up the universe, and a field for every force that defines how those particles interact. We can't see the fields, but we can observe the results of those fields. The universe is like a boiling pot of water full of energy, and we only observe what is on the surface.