What a bummer - to send an intergalactic probe to a far-off supercluster only to find it’s made of antimatter and you cannot interact with it (if you survive the close encounter long enough to find this out).
If you are paying attention, and on a trip of that magnitude I think you would be, you would notice the halo of gamma rays as the antimatter supercluster interacts with normal matter around it.
That said, the Larry Niven short story “Flatlander” is pretty funny. You should read it.
