No one can be certain, because no one can see the entire universe. But so far, the farthest star from earth that scientists have found is about ten billion light-years away.
Since one light-year is equal to six trillion miles, then the farthest star from earth that we know about is around 60,000,000,000,000,000,000,000 miles away!
But the universe must be much bigger than that. Suppose that the earth were at one edge of the universe and the very distant star was at the other edge. Then the universe would be ten billion light- years across.
But we know that the earth isn’t at the edge of the universe, because we can look in all directions and see stars that are many millions of miles away. So all we know for sure is that the universe is at least ten billion light-years across!