Gravity

You may have noticed that none of the gauge bosons seem to carry gravity. In fact the standard model doesn't include gravity. The reason that the standard model is able to predict particles so accuratly despit this, is because of gravity's low strength in the subatomic realm. Also, the theoretical particle for the gravitational field would be called a "graviton"

Gravity is the weakest of the four forces. In fact, gravity is much weaker than the other forces, being 10^24 times weaker than the weak force.

Also, our best method of describing gravity is from Einstein's theory of general relativity. In general relativity, objects warp space-time, which affects the natural motion of objects rather than gravity being a simple attractive force.

This is a great video that I think demonstrates the concept well. He ends his explanation around 4 minutes

While general relativity and quantum mechanics both explain motion extremely well, there have been many problems in combining the two theories

One problem is with the concept itself. We derive quantum mechanics by quantizing forces, using space-time as a continuous background. Because gravity is the changing of space-time, doing the same thing is much more difficult. We would have to quantize space-time, meaning there is no clean coordinate system that we can use as a background.

This itself leads to even more problems. For example, in general relativity, the warping of space-time is caused by mass and energy. In terms of quantum gravity, gravity would be an excitation of quantized space-time. This means that the energy from excitations means more warping, meaning more excitations, meaning more warping, to infinity.

While this self-interaction is seen in other forces, it can be fixed using something called perturbation theory. Perturbation theory basically makes complex interactions into simple interactions using corrections, which sometimes use real-life measurements to bring the math back to reality.

However, this doesn't work for gravity. There aren't any simple measurements we can perform for corrections and it is impossible to precisely detect particles below the Planck scale.

In order to interact with a particle, we need to interact with it. Usually, we do that by bouncing a photon off it. If we want more precision, we need more energy in the interaction. We can use the Heisenberg uncertainty principle to find the energy needed for a certain amount of precision of position. In order to locate a particle with precision below the Planck length, we would need enough energy to create a black hole. However, this doesn't happen, and black holes aren't spontaneously created, meaning that there is something wrong with our current models of physics.

Overall, there are many theories that try to create a theory of quantum gravity. A theory that combines our understanding of quantum meachics with gravity would typically be called a theory of everything.