The cosmic micro-wave background (called the remnants of the "Big Bang") is very uniform. This is (naturally) what you would expect from a big bang (if it ever occurred). However galaxies are not uniform. They are very large, are said to have developed early on after the big-bang, yet they're so "clumped". This contradicting fact all but completely sing-handedly disproves the big-bang theory.
The rate-of-expansion of the universe is also used to claim it is the result of the big-bang, however it hasn't been shown to be in the right ballpark for the expansion after a big bang. It is also expanding so much that it will never collapse on itself, which is what completely disproves the "big-crunch" theory. Funny how they'll let that disprove the big crunch, but won't let a little thing like non-uniform galaxies get in the way of their big-bang theory.
"it could be possible that at some point in the past there was a virus which would fuse itself with the cells DNA rather then take full control of the DNA it would would become a part of the cell and change it"
Shimraa, you're talking about things that don't exist, and there's no evidence of them existing either. You're right, the fundamental form of life is the cell. Even ignoring that, a virus does not perform the necessary functions of life. It is a non-living thing. It performs operations out of necessity only.
Anyhow, I’ll explain, if I must, to you why such a virus could not exist. Because it wouldn't be able to reproduce, end-of-story. If it fused with a cell, then it can't infect other cells, it can't spread. I find the idea of a single cell's DNA changing effecting the whole completely ludicrous.