I think the title says it. Did expansion of the universe steal the energy somehow?
Answer
Energy isn't a nice concept in GR, so all I'm giving is an intuitive way of looking at it.
For gravitationally redshifted stuff: A photon has energy, thus it gravitates (as energy can gravitate analogous to mass from $E=mc^2$), thus it has some (negative) gravitational potential energy when on the surface of a planet. If it's emitted, its GPE eventually becomes 0. So, this increase in GPE had to come from somewhere: the photon's redshift gave the energy. It's pretty much the same thing that happens when you throw a ball up. It loses kinetic energy (slows down).
The GPE in relativity is basically related to the energy stored in spacetime curvature; in a complicated way that I don't know.
For a normally redshifted photon from a moving body: Energy need not be conserved if you swith frames. Energy is different from each reference frame.
See the answers to the question provided by Qmechanic above as well. Over there, they're talking about the entire universe, though, which leads to additional issues.
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