Kryptonians are another modified human race, physically similar to the human base stock but with internal differences.
Designed to populate a high gravity Terrestrial world with ample mineral resources but little fertility, the Kryptonians were gene-engineered to take a lot of their sustenance from solar energy, much as their creators do. In an effort to be economical, the Galactics also engineered the Kryptonians to manifest much greater energy control powers when they were bathed in the rays of a more powerful energy source, such as a more powerful star or the close presence of a Galactic.
Apparently, the Galactic who engineered this offshoot of humanity either lost interest or met with a mishap without informing any of his fellows. The Kryptonians were left alone rather than constantly raided to supply high energy workers/agents for Galactic purposes.
As far as is known, the only Kryptonians to be exposed to a higher energy source were Kal-El and Kara El, cousins who were each sent off to Earth just before Krypton exploded (in Kal’s case) and after Krypton exploded (in Kara’s case) when the moon colony on which she was born became uninhabitable from Kryptonite radiation. Under the stronger rays of Earth’s sun, each developed powers that made them Superman and Power Girl. While seemingly physical paragons, there is some theory that they are, in fact, unconscious controllers of gravity and other quantum forces.
Daxamites are apparently an offshoot of Kryptonians, products of either the same Galactic or someone following similar lines of experimentation. In many ways a more successful experiment, their powers are even more dependent on a constant supply of radiation. Anything that blocks that depletes their energy reserves and taps into their life force. They are not affected by this condition before they are activated by exposure to a stronger sun, but the interaction is deadly afterwards.