When asked why I don't believe in a god, the best explanation I've been able to come up with is to point out how we are constantly learning how the universe works and none of the discoveries of the last century has been that there is a supernatural being waving a magic wand. I explain that since the bible was written all those years ago, man has discovered that disease is caused by germs and bacteria, not demons or an angry god (2 Chronicles) as the bible claims. Scientists have discovered that the Earth is spherical not flat as the bible claims (Isaiah). We now know bats are mammals not birds as the bible claims (Leviticus). Scientists now know that the universe does not revolve around the earth as described in the bible.
We know all this because scientists formed theories and then performed experiments to test those theories. When experiments provide evidence that a theory is correct it strengthens the theory. When evidence proves that a theory is wrong, then the theory must be adjusted or discarded. That's the scientific method. Science is testable, repeatable and can be independently verified. When a scientific theory is disproved, scientists don't make excuses, they don't have 'apologists,' they go back to the drawing board, they examine the evidence and develop new theories.
Science is always discovering more about how our universe developed. Because we know the speed of light, we can actually 'see into the past' when looking into space over great distances. I am constantly awed by what scientists have discovered and are continuing to learn about the universe we live in. To me, saying 'God did it' is a cop out. It's like saying, "I'm not smart enough to understand all this physics stuff, so I'm just going to attribute everything to an all-powerful being."
I realize that many people, even some scientists, feel the need to believe in a god in order to impose meaning on the universe. They want to answer the question, "Why?" I don't necessarily believe there is a 'why' to the universe. It exists. There doesn't need to be a purpose for the existence of the universe, or our galaxy, or our solar system, or our planet, or animals, or humans. I am comfortable just existing. I make my own purpose.