There could be more to the universe than the three dimensions we are familiar with. They are hidden from us in some way, perhaps because they're tiny or warped. But even if they're invisible, they could affect what we actually observe in the universe.
There could be a vestige of extra dimensions hidden in your kitchen cabinet.
It's not completely obvious what gravity is, fundamentally, or what dimensions are, fundamentally. One of these days we'll understand better what we mean, what is the fundamental thing that's given us space in the first place and dimensions of space in particular.