(1) the observed differences in gravitational forces come from a gravitational field source, not modified equations themselves

(2) The field sources are a form of matter, thus obeying some "obvious" laws like mass conservation

(3) The new kind of matter is also attracted by gravity itself, which even allows for limited predictions of how its mass distribution behaves

And we can only say it's never been observed directly, because there is indeed enough indirect evidence pointing towards its existence.