I don’t really know what this question is going for. If matter interacts gravitationally then it still exists in 3D space.

Is this a String Theory market? If so, even then the extra dimensions matter would exist in aren’t exactly “outside” 3D space.

@ConnorDolan Or do you mean inference on the existence of matter outside the horizon of the observable universe? It’s very plausible in that case that there is, but impossible to ever observe evidence for it.

@ConnorDolan "if matter interacts gravitationally then it exists in our 3D space" -> it's possible that this statement is false

Does quantum interference with other Everett branches across Hilbert space count?

@TheAllMemeingEye could you go into more detail about what you mean?

@RemNi my understanding of the Everett Many Worlds interpretation of quantum mechanics is that, because the wave function never collapses, it forms an infinite series of parallel universes across the infinite-dimensional Hilbert space of possible functions, which could be interpreted as additional dimensions to the 4 convectional space-time ones. If decoherence hasn't taken place (e.g. due to physical interaction/observation), then you can get parts of the wave function constructively/destructively interfering with each other (e.g. Young's double slits experiment), which following the Everett interpretation is equivalent to the particle interacting with other versions of itself from alternate universes outside our 4D space-time. Thus, if scientific consensus on the Everett interpretation reaches 90%, would this resolve yes immediately?

@TheAllMemeingEye I'll give this some thought and get back to you

@TheAllMemeingEye I assumed that wouldn't count, so in case @RemNi says it does I'll sell all my NO shares

@ArmandodiMatteo I'd prefer to give this a bit of thought before replying.

@TheAllMemeingEye my thinking with the Everett interpretation is that it is unfalsifiable, and (for now) doesn't predict novel phenomena in the universe. This makes it scientifically weak as a theory, and it's unlikely to convince 90% of the scientific community for this reason.

I can maybe add some details to the question emphasising this. To be clear, if there was a way of testing the Everett interpretation in some lab experiment, and the results led the scientific community towards a consensus supporting the presence of matter in other universes, that would qualify.

@RemNi right, thanks for clarifying