> Here is a rule for any armchair astrophysicists: whatever you think of, that was most likely an idea at one point and was eventually ruled out.

The relevant XKCD is Astrophysics - https://xkcd.com/1758/ ( https://www.explainxkcd.com/wiki/index.php/1758:_Astrophysic... )

The transcript from explain:

    [A sign on two posts, in the grass in front of a building with windows and double doors, a window on each door, and bars facing outwards. There is a cement walk leading to the doors. On the sign is the text:]

    Department of Astrophysics
    Motto:
    Yes, everybody has already had the idea, "Maybe there's no dark matter—Gravity just works differently on large scales!" It sounds good but doesn't really fit the data.