Not my idea. Blame Bear
I would assume it's not an invisible photonic dimension so much it is another potentially visible plane of existence - like using VISR mode, but on the scale of Slipspace. The Didact used it to track otherwise difficult-to-see movements during ship-to-ship combat if I'm remembering right. It's not one of my baked-ass ideas though.
Hmmm... I guess the issue arises from how the lore describes slipspace and methods of entering it. The books mention the Shaw-Fujikawa drives access through the use of particle accelerators to create small black holes through which they somehow squeeze the ships. Now, even though these black holes evaporate due to Hawking radiation extremely quickly - which, in this state, includes the mass of the ship - the ships are not destroyed. Instead, the lore goes along with a somewhat fringe theory of physics that black holes destroy the laws of physics to basically allow access to parallel universes of space (although they tend to use the words dimensions, universes, space, etc. interchangeably, which is quite annoying).
Ok, so let's go down this road then. This means slipspace is essentially a compressed, parallel universe running alongside our own, which leads to the issues with relativity. Even in this case, you are still dealing with matter / energy interacting with space, just a
different space. This leads to my issue with the proposal above. Since the syntax in the lore is mixed up without a care, this "plane of existence" is either another dimension or another universe (like slipspace). In the first case, a dimension doesn't necesitate spacial properties perse (like the ability to have any recognizable dimensionality), but in order to have particles (photons) present, the energy waves have to interact with a field, which requires some interaction with other dimensions for the particle's existence to be perceptible. This can't be the case, otherwise it would be observable in one or more other dimensions (dimensions with purely energetic existence are possible, but particles require field interactions). The other possibility, yet another universe, again requires that the electromagnetic field be present for interactions to result in photons. This means that fundamentally, photons are not the only thing that the universe is made of (they are a side effect of a field's existence). They can also not be the only thing taking up the space in the universe because the field needs space itself in which to exist.
Of course, this could all be shoved under the rug if they just came out and said "these other universes follow different laws of physics so these things are possible". But then that leaves the nagging issue that apparently matter/energy/field interactions from our universe can safely enter, traverse, and exit slipspace with fairly good accuracy separate from time. This requires us the crazy small chance that the slipspace "universe" is essentially empty space but just happens to be habitable.
Then again, Master Chief is THE ONE, so I suppose who gives a flying fuck anyway?
/YourExWahrer
