Why is the universe thought to be infinite?
First, after the Big bang, the universe started to expand rapidly and due to research done in past centuries prove that our universe is accelerating faster than it did at the time of Big bang. Secondly, as there is no fixed centre of the universe, it is thought to be Infinite(although there are many types of infinity and I may be over exaggerating here). Currently the universe is measured to be 13.8 billion years in age. So it must be Infinite as far as we know it. Who knows what crazy physics will come out in the future?