It is because anything with mass in space produces a force of gravity that draws things to its centre. This has been proven in experiments (although flat earthers would argue that these have been faked - although they won't be able to come up with a good reason as to WHY they should have been faked).
Which experiments are you referring to that have shown that
anything with mass produces this "force"?
A theorised reason as to WHY it produces gravity is a bending of the fabric of space, and is the 3D equivelant of putting a heavy object in the centre of a trampoline - everything else that is lighter on the trampoline will roll into the centre because the fabric of the trampoline slopes in towards the heavy object.
<<dons RE hat>>
omg111, I hate this analogy. If you do this stupid trampoline experiment in deep space, will the lighter objects roll to the centre? Hells no. They only roll to the center because there's
already gravity there (I refer of course to the Earth in its upward acceleration).
Take away the gravity and even if you find some other way to bend the sheet, objects will not roll "down" towards the centre.
(Hint: "bending space": not interesting.)
<<dons FE hat>>
That's a nice theory, but it's no more satisfying than this mysterious "force" you mentioned in the first paragraph -- you've answered the question, "What exactly is this force of which you speak?" by saying, "Well really it's just the bending of space," which of course raises the response, "Okay, let me rephrase: why does mass bend space?"
-Erasmus