AOA!
The Theory of Infinite:
To understand the infinite property of universe using high level physics is not reasonable so I'll discuss only in simple way
Let's start
See this picture
Let a ball starts moving from point A anticlockwise
After passing through B,C and D points it comes again at point A
If ball does not know about the area other that the circular line it'll unable to determine that it is infinite or finite
Same is the case for the universe
Universe is circular in forth dimension or Time
We can only determine that universe is circular in fourth dimension that
If an object goes in one direction then it returns at starting point
That's why we seem universe an infinite
Another way to determine that universe is circular in 4th dimension is that super clusters are at organized specified distance and like the object between two mirrors paralleled to each others
<Thanks for reading if any question then you can ask!>