In a Nutshell : The universe is expanding because it is filled with more dark energy than matter.
At the Start …
According to the big bang theory, the universe started off as an infinitely small point and expanded incredibly quickly in a process called inflation. To go from essentially nothing to about the size of a Space Hopper took just 10-34 seconds (that’s 0.00000000000000000000000000000000001 seconds!). This early universe expanded at 3 trillion trillion trillion times the speed of light. The original cause for the expansion of the universe was inflationary pressure from the big bang, thanks to which the universe is now about 150 billion light years across, even though it is only 13.7 billion years old.
As a result of the big bang, lots of matter appeared in the form of gas, dust, stars, galaxies, black holes, and so on. Matter causes gravity, and gravity is a force which attracts things together, so if the universe were made only of matter, then gravity would be acting against the outward pressure of big bang inflation. Astronomers would expect to see the expansion of the universe slowing down, or even going into reverse, but in fact they see the expansion accelerating. This strongly suggests that the universe must be stuffed full of some sort of energy, but the problem is that no one can see this energy and no one knows what it is.
In The Dark
All the visible matter in the universe, including all the galaxies, stars, planets, black holes and nebulae, adds up to just 5% of the total
Scientists call this energy dark energy. They know that about 70% of the universe must be made of dark energy, and another 25% of a form of invisible matter called dark matter. All the visible matter in the universe, including all the galaxies, stars, planets, black holes and nebulae, adds up to just 5% of the total. The dark energy is somehow causing the universe not just to expand, but to expand increasingly fast.
How do astronomers know how much stuff forms the universe ? : They measure something called the cosmic background microwave radiation (CBMR), which is a remnant of the big bang, like a very faint echo. The CBMR accounts for 99% of the radiation in the universe. You can see it for yourself by switching on your television set but leaving it untuned- about 1% of the static you can see is your television aerial picking up the CBMR.