How do we know that the effects of "dark matter" are not merely a failure of some other aspect of general relativity? Could it be that it is not matter at all but instead some placeholder for another aspect of our universe, or is it proven that there must be some form of 'matter', so to speak? If so, what is this proof? I understand that there is some forms of radiation which we predict results from dark matter interactions, and I admit I don't have full understanding of this topic, but I am curious as to why dark 'matter' is a popular theory, and not some other explanation for the discrepancies we observe. Thanks.