It depends what answer you're looking for! We can tell you how photons behave, how to recognize them, and how to predict what they do. For hard philosophy questions, that's more under sections 100-119 of the Dewey Decimal than something physics aims to answer
>what medium is present in a vacuum that allows things like gravity to propagate
That's a little like asking who is in charge of pushing the sun around the Earth. There's a presupposition in that question, you should first ask why you expect there to be a medium, besides nondescript vacuum
It depends what answer you're looking for! We can tell you how photons behave, how to recognize them, and how to predict what they do. For hard philosophy questions, that's more under sections 100-119 of the Dewey Decimal than something physics aims to answer
>what medium is present in a vacuum that allows things like gravity to propagate
That's a little like asking who is in charge of pushing the sun around the Earth. There's a presupposition in that question, you should first ask why you expect there to be a medium, besides nondescript vacuum