Imagine a lake. Peaceful, calm, beautiful. There are trees, acting all tall. The sun's shining down. In the distance mountains chill out. Then suddenly, Splash! Something hits the water. Is a child hiding and throwing rocks? Was that a meteor?
Now picture yourself in the ripples that were created. Now picture yourself as those ripples. Imagine that if you zoom in really really close to one little ripple you see a little droplet of water rising out above the rest. That's my analogy for the sun. There's an even smaller droplet of water beside it, and that represents the Earth. And if you zoom in even closer there are a couple molecules of water. Those represent you.
This tiny aquatic world is different then our own, of course. It's two dimensional. The water itself takes up three dimensions, but most of its just solid. The interesting part, the surface between the water and the air, is two dimensional. If you walk far enough in one direction, you really would fall off the water Earth.
Now imagine little water you looking into a little water telescope. You see all the little water stars extending out as far as you can see. And as you watch one, you notice its getting farther away. Intrigued, you look at another star. Its moving away. Another star? Leaving you! Another star? Same deal! Pretty soon, you're positive all the stars are moving away. This ripple is traveling towards the shore, and as it does so, its radius is getting bigger.
So you take out your water calculator and start thinking. You figure out how fast the other water stars are moving away. You figure out how far away they are now. And from that, you realize that all these stars must have been at the same place some long time ago. You do the math and see, wow, that was 34 seconds ago!
Now you've got an idea of when this watery universe started. The very first moments wouldn't be clear: Everything would sort of converge to a ring of a certain size. If you could study the stars enough, you might figure out the contours of that ring. Maybe its smooth with a few ridges, like a baseball. Maybe its more irregular, like a rock. What was it that caused the universe?
Maybe water you would theorize that perhaps there's a third dimension, and something collided with your two dimensional universe. But ultimately, you could never know what that thing was. It's deep under water now. With all the information in the surface of the lake, you'd only be able to construct a 2d cross section of the object. It was three dimensional and left, and you're only 2d. And even if you could guess at the shape of the thing, it wouldn't tell you about what set it in motion. Did it fall of its own volition? Did somebody toss it?
Now stop imagining, and return to our world. What caused the big bang? What existed before it? If there are more dimensions than 3, can we explore them? Will we ever know these answers?