Found it.
Ratbert starts with "So, I'm thinking what if every proton is just a densely packed universe, and to them, our universe looks like a photon". The rest of that strip has an inimpressed Dilbert, but Ratbert continues in the next strip by talking to the Garbage man:
Ratbert: "So... each photon is a universe... then mass is just a probability clister?"
Garbageman: "That's how I see it."
Ratbert: "Wow! I think my tiny skull is so full it's going to explode"
Then there's a gag with Dogbert asking "Have you been talking to our garbage man again?"... But in a third strip:
Garbage,an: "... as your conciousness passes through each universe, you tend to follow a line of probability. And since it's more probably that matter is near other matter, you have the illusion of gravity as your conciousness moves towards the norm."
I know it's meant to be far-fetched, but it sounds plausible and I like it for the easy way Scott Adams described it.
Wade.
"Ah. One of the difficult questions."