Before the explosion, everything in the universe was contained in a point. The point had a spin to it. Because of the gravitational forces contained within the point caused the explosion which occured 15,000 million years ago. All matter, energy, space, and time were created. Within minutes of the explosion, atomic particles came together to make the gases helim and hydrogen, which over millions of years produced the galaxies, the stars, and the Universe as we know it today.
Scientists believe the universe has been expanding ever since.
If you hold your finger in front of you and look at it first with just your left eye and then just with your right eye, your finger shifts position against the background. The closer your finger is, the bigger the shift. The shift is a measure of the distance between the finger and the eye. This effect is known as parallax and, on a grander scale, it can be used to calculate the distances of nearby stars. As the earth orbits around the Sun, a star will appear to move slightly against the background of more distant stars.