and your comment: > Imagine some kind of alien life form that arose and evolved in intergalactic space.. but they have never seen any sufficient concentrations of mass to notice gravity.
I am wondering is it possible that we also evolved in some "special" part of space in which we do not percive some other ("fifth") fundamental force of nature just like your 'intergalactic aliens' do not know about gravity?
For example, we only knew basic details about the strong nuclear force from the 1910s, and the empirical ingredients for the weak force weren't around until the 1930s.
We still have very little idea of what mediates dark energy, as well as dark matter interactions (potentially, depending on your favorite model for DM). We know they interact gravitationally, at least.
Edit: I meant to say it's something of a cosmic accident that we don't interact more or a lot less with the nuclear forces. We are 'lucky' we get to interact with electromagnetic forces so easily.
Thank you. I am always looking for decent sci-fi recommendations.