The universe has no center.
AT THE SAME TIME
Everywhere is the center.
Confused? Yeah, it's confusing. But consider that there is no center because there are no boundaries to the universe. There is no wall at the far end that you reach if you head in one direction for a long time.
If you were to head in one direction for a loooooong time, you would eventually get back to where you started. It's kinda like how if you take a plane on Earth going west you'll eventually get back to your starting point.
Basically, the universe loops back around on itself. Therefore, the shape of the universe cannot really be understood by us visually because it is not a 3D shape. It has more dimensions.
But the bottom line: there is no center because everywhere is the center.