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I think Dyson swarms are cool, but they seem pretty impractical though. Matter antimatter reactions have always interested me but i have yet to look into them to a great extent.

I got pretty excited about them until I realized just how much energy is required to create, separate and losses/danger in holding those antiparticles. But once you get to that point rather fun as a fuel source. I like the idea of the swarms however, as the excess energy could do some very fun stuff like Matrioshka brains, or creating an abundance of anti-matter.