Science is based upon the idea that you can do an experiment to prove or disprove things. So, for example, if I state that gene X causes a disease – I then need to do an experiment that will either prove me right or prove me wrong.
No-one has been able to do an experiment that proves any God exists, so there is no scientific evidence that supports it.
I agree with Darren and Daphne, but I would add that to people who believe in god, god is very real because they have faith that god exists. As Daphne says, science is about proof and evidence not faith.
I think it is easy for some people to think that science and religion don’t mix but religion is a personal choice and it is up to the person to decide if they believe in God once they have a chance to understand what it means.