Half Life physics

This interpretation of Half Life's multiversal physics has 3 "universal dimensions" on top of 3 spatial dimensions.

Dimensional Gravity:
A fifth fundamental force similar to gravity, that is extremely weak as it propogates across all 6 dimensions, with strength proportional to 1/r^5 instead of 1/r^2.

Grey hole: A 6-dimensional black hole with the mass of countless universes, held together by both regular and dimensional gravity. It has a greater radius in ordinary space than it has in universal space, as in ordinary space it has regular gravity which doesn't lose as much pull with distance as dimensional gravity, so the event horizon is much further out in ordinary space.
The 6-dimensional pancaking of the grey hole means that the normal of the surface on most points of it is extremely close to zero in regular space, but strongly angled in universal space.
This proves to be important when one considers the Hawking Radiation of this grey hole; although the mass of it is absurd, it has a much larger surface due to being 6-dimensional, and so is much more luminous than a black hole; hence the name "grey hole", as it is an inescapable void yet highly luminous.
This luminousity is projected more into the universal dimensions than the spatial dimensions, meaning the grey hole's radiation is not a perfect sphere.

Universal shells: Just as any mote of dust with a certain mass:surface ratio will stay a certain distance from the sun due to solar wind balancing with gravity, any fundamental particle with a certain mass:6D-surface ratio will stay a certain distance from a grey hole due to radiation pressure balancing with dimensional gravity.
Thus, particles will accumulate in shells around a grey hole, according to how easily they absorb 6-dimensional radiation relative to their mass.
All our fundamental particles have exactly the same value for this ratio, and thus our universe hovers a specific (extremely far) distance from our grey hole.
Many universes can exist inside a universal shell; anything with different coordinates in universal space is a different universe, as there is no path to it through ordinary space.

Matter recycling:
As mentioned before, the radiation around a grey hole is not a perfect sphere. For our universal shell, the area where there is almost no variance in radiation pressure is billions and billions of light years across, but as matter travels further and further from this area, radiation pressure starts to drop asymptotically to a nonzero but extremely small strength; and the distance one has to travel to get a certain percentage drop in radiation pressure is proportional to the distance one is from the grey hole.
This works out to mean that universal shells curve into the grey hole; their exact shape is extremely hard to describe, since it's a distortion of a 6-dimensional sphere.
As everything tends to drop to a lower potential energy, matter will always end up in the grey hole; moving absurdly slowly towards it at first, but faster and faster as it gets closer.

Xen:
What if matter doesn't fall into the grey hole, but instead ends up in orbit?
Then you get Xen; a 6-dimensional cloud of debris.
Since it is not part of a universal shell, the standard model of physics goes out the window; particles belonging completely different universal shells can end up being part of the same mass since they are held by orbit instead of radiation pressure, resulting in Xen displaying properties that anyone in any universe would regard as physics-defying.