Q: Is it possible to claim legal ownership over all the dark matter in this universe?

A: There is a minute of my life I'll never get back. Seriously, all space issues are governed by UN treaty, so no.

A: Well, the law of this is a bit fuzzy, but there are many precedents which I will refrain from citing until I'm retained. But, you could take title from the Creator himself, as Moses did, and thereby rely upon your title deed by Roman law of dominium by subjugating and defending the dark matter that you may possess. If the dark matter is ownerless, Roman law would also provide for an action in occupatio by the first person to take actual possession of each parcel of it as ownerless. You may need to wait until you get your own large hadron collider and it discovers some of the dark matter, and then you might move your recreational vehicle on it before filing a claim in the Roman Imperial Courts. Alternatively, Act of War has had a more modern effect on possessory rights, though those rights tend to be possessory rather than Romantic, dominium rights. In other words, like the Israelis and the Arabs, you may find that prior possessors and claimants deriving from them keep coming back to claim their "prior" rights. But, your first step there would be to move your army on to the dark matter and defend it against all claimants. You might first act to dispossess the international celebrities also possessing heavenly objects. Just remember before getting violent on this planet that your war for possession must be at the site you are claiming. You and the celebrities must plant your flags on the moon or in the dark matter, then assemble your recreational vehicles and armaments there. We've had quite enough gun violence here, thank you.

Once you've established dominium, invite me over for a scotch. I like single malts from Islay.

Gee, and Mr. Siegel only wasted a minute of his life. Sheesh.

A: I actually am the deed owner of said dark matter, I got it at a foreclosure sale in Queens, NY a few years back.