Why is it difficult to say how big the Universe is?

It is difficult to say how big the Universe is, because we can't even imagine how big it might be. Our Earth is only a tiny, tiny part of our solar system, and our solar system a tiny part of another system called the 'galaxy'. Beyond our own galaxy lies a vast expanse of galaxies. There are billions of galaxies, the most distant of which are so far away that the light arriving from them on Earth today set out from the galaxies billions of years ago! Perhaps, all these billions of galaxies put together are still only a tiny part of a larger system. So, we know that the Universe is bigger than what any one can imagine, but how much bigger is a question that Man has been asking himself since the beginning of time. The answer is still beyond our grasp, especially as scientists believe that the Universe is still expanding!