By Stephen Hawking 
Science and religion both offer answers to fundamental questions. Before science, only religion provided these answers. But science has provided a more rational, consistent and verifiable explanation: our universe is a giant machine, governed by a set of unbreakable and universal natural laws. This means that everything — even God — would be subject to them which contradicts theology’s insistence of divine omnipotence.
What came before the Big Bang?
There is a lot of scientific evidence to support the Big Bang theory. But it begs the question: What came before the Big Bang? To answer that we have to understand that the universe exists on a platform of space-time which can be warped by the gravity of massive objects. In fact, black holes can warp space-time so violently that time itself stops.
Moments before the Big Bang, the cosmos contracted into an infinitely small, infinitely dense singularity similar to a black hole. Under these conditions space and time no longer function. So we have the answer to what came before the Big Bang: Nothing came before the Big Bang because time didn’t exist. And there was no cause of the Big Bang because there was no time for a cause to exist in.
Why haven’t we encountered alien life?
There are 40,000,000,000 Earth-like planets just in our cosmic neighborhood that could support alien life. So why haven’t we been visited by any of them?
On Earth it took 6 million years to go from the beginning of life to the multi-celled organisms needed for intelligent life. An asteroid that could wipe out the vast majority of animal life is expected to occur once every 20 million years on average. If this is true on other planets, then an intelligent species has to have some luck not to get wiped out before they have become sufficiently advanced.
The amount of time it takes from beginning of life to a sufficiently advanced intelligent species is a significant span of time in the life of a planet’s sun. It’s possible that many intelligent aliens have been wiped out by the natural death of their sun.