This is simply untrue. We have a well tested theory and an explanation.
We have many theories, most of them untested (and of these not many are even all that well formulated). However, restricting ourselves only to the well-tested theories explaining gravitation and/or fundamental forces, we immediately run into the problem that
1) The most used theory of gravitation is a modification of Newton's theory in which gravity is an invisible force field defined at every point in the universe. The description of Newtonian gravity as a
field would be quite alien to Newton as the concept was developed as a crutch by Faraday to describe electromagnetic fields and later formalized by Maxwell and others. On the one hand, defining Newtonian gravity as a field theory automatically makes it local (whereas Newton's gravity was so incredibly nonlocal it makes even the most fantastic descriptions of quantum nonlocality appear mild). On the other, there was no justification for it.
Worse, we know that Newtonian gravitation is not adequate
even as an effective field theory within the scales it was developed to operate at. To the best of my knowledge, we still have not discovered the planet Vulcan that Newtonian gravity predicted but which general relativity does not.
2) General relativity has so far proved to be very well tested for the most part both in successfully giving us all the correct predictions we'd obtain from Newtonian gravitation and succeeding where Newtonian gravity fails (namely, in explaining the motions of things like planets as well as larger systems). However, general relativity explains gravitation as a dynamical interaction between the physical system of spacetime and what we would classically think of as physical systems in space and time. Simply put, in general relativity gravity is not a force. Gravitation is just a term we give to the effects that result from the manner in which systems with energy change the local geometry of spacetime.
More importantly, though, general relativity completely fails to predict anything or even be compatible with anything that correctly predicts the behavior of atomic or subatomic "particles". In the standard model of particle physics as well as just about all effective quantum field theories (and quantum mechanics), we run into problems even with special relativity. But in special relativity, space is affine. So one can still quantize (canonically or otherwise) classical systems/forces and arrive at e.g., QED or the standard model or whatever. However, there is no way to quantize systems (canonically or otherwise) without doing so by treating the systems as somehow described in spacetime. Simply put, we can't treat systems quantum-mechanically without doing so by describing them in spacetime, and general relativity requires that they be treated as interacting with a dynamical spacetime not quantized within one.
3) In particle physics, we have the graviton. One can go to the Particle Data Group or to standard references and find that the spin of the graviton, that it is a kind of boson, and so on, and yet there is absolutely no empirical evidence for any such entity. It is not even entirely clear what form such evidence might take. Mostly we just take what what must be true of forces in the standard model apply this to what we would like from quantizing gravitation even though this doesn't actually lead anywhere.
So, we have the most familiar form of gravity provided by Newtonian gravitation that has been thoroughly tested and so we know that it breaks down and we know at least at large-scales how and when it does. We know that this force doesn't exist as more than a useful mathematical model. The best theory of gravitation is then the theory of general relativity, in which gravity isn't a force. This highly successful theory is completely at odds with the entire nature, structure, and framework of modern fundamental/particle physics. Then we have the theory of gravity in particle physics: there is a particle that doesn't exist but if it did we could say some stuff about some properties it would have to have given the standard model.
