>>7598375 lb
Newton and Einstein give us equations for four forces.
We can ONLY test them from our position in the Universe, we can only make observations from where we are. We assume the Copernican principle that states our position is not special, we do not if this is true, since our position has some special characteristics (like being close to a star).
When we look at the rotations of every galaxy we can observe from here and count up the visible mass in each, the further from the centre of the galaxies, orbits do not obey our theories, i.e. the inverse square law does not apply far out from the centres of the galaxies and gravity appears to be stronger than it should.
A fifth force that acts differently than our theories at long distances and/or under low gravitation, could explain this, instead of or explaining the bullshit we make up as dark matter or dark energy.
TL;DR none of theories match long distance observation, this might help