Disclaimer: I'm not an astronomer, physicist, mathematician, etc. so this is a question from a complete newbie.
One of the greatest mysteries of our age is "where is the dark matter?" The universe can be observed to be curved (aka have gravity) in many places where we cannot detect any sufficiently large mass nearby.
There are many hypotheses about this, but one I've never seen discussed is that the universe simply isn't flat by default. The assumption always is that gravity is the only thing that curves the universe, and that without any mass there will be no curvature. But what if it isn't true? What if in it's "default state" the universe isn't smooth and flat, but has some random curves anyway, just like that?
Of course, this is just another hypothesis, but my question for this site is - Why doesn't it seem that anyone is considering this possibility?
It seems I haven't explained myself clearly enough. To put it in analogy - mass & gravity are often depicted as objects weighing down on a sheet.
Dark matter is the same, except we cannot see the "apple". The bend is there, but there's nothing in it. We can see other objects attracted to it, gravitation lensing works as expected, etc. But nothing there when we look at it directly. It's like an invisible apple.
And my question is - why do we assume that the "sheet" is smooth by default? That there cannot be bends in it without apples. Or, in other words, yes, we know that mass causes gravity, but why cannot gravity ( == bends in spacetime) also exist without mass?
Added 2: It's also quite possible that my pop-sci level understanding of gravity is way off and I'm asking nonsense. In that case, I'd like to understand what I'm missing.