Yeah, they could just ship hydrogen back. They could also beam lasers at us although I image it would be awfully wide by the time it got here.

Also, so as long is the energy to ship it is less than the energy to create it by fusion, it would be economical to ship elements heavier than iron. But that would mean annoyingly slow speeds.

The really annoying thing is that by adding trillions of people to the universe, we apparently won't really improve the rate of idea creation. I really hope I'm missing something on this.