The universe is infinite. I don't think even the smartest mind on the planet can possibly imagine the depth of what is infinite.
Think about every star, every solar system, every single cluster of galaxies and the space between them. I believe even that mass comes to an end, and space just carries on.
Its easy to imagine something having its limit, we humans create limitations in our mind for a reason. But is it so easy to imagine what goes beyond that and never ends?