[nyssos]

They are not. MACHOs definitely exist, it just turns out there aren't nearly enough of them. Axions were proposed as a solution to the strong CP problem years before anyone went looking for dark matter candidates. Sterile (i.e. right-handed) neutrinos are motivated by the need to explain why left-handed neutrinos, contrary to the predictions of the standard model, have mass. Supersymmetry was originally an attempt at strong-electroweak unification.