It’s a very tough question because we know surprising little about gravity compared to other forces in the universe. There are a couple of ideas I’ll explain to you though:
Einstein’s idea – matter is able to bend spacetime around it and this bending of space is how gravity works. If you imagine stretching out a sheet of rubber over something, then place a heavy ball on the sheet, it would sag under the weight of the ball, something like this:
Then anything coming nearby would roll down the sheet towards the heavy ball bending it. It would get ‘pulled’ towards the heavy ball – just like gravity. So matter exerts a gravitational force by bending spacetime.
Another idea is that matter is able to make particles called gravitons that fly through space and tell objects to be attracted to each other by gravity. We know this is how all the other forces in the universe work (there are only 3 more: electromagnetism, strong and weak forces). We *think* gravity should work in the same way – we even know exactly what a graviton would look like. The problem is, even though we know what to look for, we haven’t ever found a graviton. That’s why gravity is so difficult!
Our current thinking is that there are four fundamental forces and various experiments have confirmed most of these apart from how gravity really works.
All matter is believed to exert a gravitational force but it is still just a theory because we are not able to measure the gravitational pull of really tiny particles. Well not yet anyway!
The large hadron collider (LHC) in Switzerland is looking at this very question. One possibility is that we don’t feel the full effect of gravity because part of it spreads to extra dimensions or parallel universes. If we can find evidence of particles that only exist if extra dimensions are real then this would support this theory!
The LHC is smashing particles together at crazy unimaginable high speeds to try and see if these particles exist. But it has to measure them quickly as they disappear almost as quick as they are formed!