First Referenced:
Our Final Acts
Inter-reality Dust is the remnant fragments, or ashes of a massive number of realities destroyed by the flailing of The Mad God. It is directly responsible for The Crushing Fist, as it was attracted to and coalesced around the Outer Shield, resulting in the increase in inter-dimensional pressure.
Cristina Thompson was able to remove the threat posed by the inter-reality dust by using the eight remaining Branch Worlds as magnets for the dust as they were ejected from within the Outer Shield. While this resulted in the utter destruction and implosion of the Branch World realities, it formed eight large clumps of inter-reality dust in the multiverse, which have continued to attract and gather further dust.
The inter-reality dust was also able to be detected by Noah Fulmer while traversing The Raving In-betweens, areas outside normal space through which other realities could be directly accessed without the need for a Portal. He described it as a glittering dust which clung in a fashion similar to tar, making traversal very tiring and difficult.