According to ChatGTP, "In physics, a force is a push or a pull that can cause an object to move, stop, or change direction."
More generally, a force is the fundamental cause of an event — any event — in the Universe. For instance, the program of Unified Field Theory is to reduce the explanation, and hence cause, of all events to be the interaction of a single, or a set of, fundamental fields.
Therefore, the so-called existence of Dark Energy is the theoretical proposal of a new "force field." The basis of the Higgs Mechanism in explaining what gives rise to mass is the proposal of a Higgs Field.
These fields are supposed to interact with matter/energy particles to cause certain effects. The dark energy field interacts with matter to accelerate the expansion of the Big Bang explosion, since it would otherwise (so the theory goes) be slowing down. The Higgs field is supposed to interact with matter/energy to cause it to resist an acceleration; that is, the Higgs field interacts with matter and gives it mass.
So despite their best efforts, modern Theoretical Physics is moving in the opposite direction of the program of Unification. We now have two more proposed "force" fields to try to explain the observed data.
This is definitely the wrong direction.
Proving the Relativistic Energy-Momentum relation, the hard way and the easier way.
This adds the proof for the relativistic mass effects from motion, proving that gravity causes it.
Two reference frames in motions are not symmetrical. A system at rest on the surface of a planet moves faster in time than a moving system. This is not the case using just Special Relativity. This is proven just using the gravitational metric field in General Relativity.