Gravity is strange. I get that matter seems to be attracted to matter. That's seems simple enough... but Why?? Every time I look at the ball on the trampoline, all I really understand is that the ball weighs down the center of the trampoline. I understand that this is an analogy, but it is of little help to my understanding of gravity, which doesn't seem to act like the trampoline or the ball. I do award creativity points to the guy that came up with that, because I sense that it must be a clever analogy, even if I don't understand why it's clever.

Short explanation. Imagine that everything moves in a straight line, at constant speed, but not in the same directions (inertia). In a flat 3d space, all the trajectories are straight lines. Imagine a sphere. A straight line on the surface of a sphere, is a great circle, not a straight line. It is curved. So in a non-flat 3d space, all trajectories on average are non-straight lines. Then take that idea and apply it to 4d space-time. It isn't Euclidean, even when flat, because time isn't the same as space. But in flat space-time, trajectories are still straight lines, even if they behave funny in Special Relativity. Finally, extend from flat space-time to non-flat space time. At any given point in space-time (potential event location) you can draw an imaginary 4d spherical surface of a 4d sphere that describes the non-straight lines that form the previous straight lines that go thru there. in all spaces called geodesics. This imagery 4d spherical surface, varies as you go along the path of a test particle. The reciprocal of the radius of this imaginary sphere, is the local curvature. In a flat space, this radius is infinity, so the curvature is zero.

Differential geometry (study of fancy surfaces and paths on them) only works for GR, not for QT. In QT there are no test particles and no trajectories. And since gravity is so weak, even a mass the size of the Sun, doesn't do much gravitational lensing. And I have known this with full mathematics since I was 16.