Hello. I'm still only an undergraduate studying physics, so there's still much I don't know. Hence why it's probably likely that I can't figure out the answer to this question myself. Anyway, I've looked into the subject of dark energy a little bit and from what I've seen we still know very little about it. From what we've observed it seems the general thought is that the universe is accelerating in it's expansion. So my question is: How do we know that gravity is not the cause of the expansion? From all the places I've read about people suggesting this idea, I've seen them told that research has shown that gravity can't be the cause. Yet, I've not seen an explanation of why this is the case. I was just hoping someone could give me an explanation of why we know, with very little doubt, that we don't have the equation for gravity wrong. Why it can't be that distances within a galaxy are too small that the "other part" of the equation of gravity is so small it is neglected, but when it comes to intergalactic distances it causes a repulsive force. Like I said before, I am not trying to present this as a theory, but just trying to get a specific explanation of why it is not possible, because I have not yet found one. Thank you for your time.