The Omegahedron explained...
This is entirely speculation and totally non-canon, but I'll give it a try...
Argo was Krypton's neighboring planet. When Krypton exploded, Argo was caught in the blast and fragmented. Argo City (where Kara lived) drifted thru space on a meteor. Without a sun, the city couldn't last long, so Kara's mentor, Zoltar, built a power source, the omegahedron. Made from a collapsing star, the omegahedron was not only a power source, but also provided light and warmth to the city. When it was lost, (more like sabotage, not accidentally dropped into space) Zoltar was blamed and exiled to the phantom zone, the city officials confiscated his notes to try and build a backup power source. The city had maybe a few days at most before the power finally shut down. To save the city and clear her mentor's name, Kara stole Zoltar's experimental ship and follow the omegahedron to Earth.
Selena was the illegit daughter of a Metropolis businessman and a gypsy woman. Mother died when she was very young and Father never wanted her, so she grew up in a British boarding school and never was close to anyone. After her father died, she inherited his fortune and took over his business. But her true passion was magic/sorcery. She's studied the dark arts and was able to cast some powerful spells, but then she found a small trinket that seemed to enhance she power to almost olympian proportions. Believing the object (the omegahedron) to be magical in nature, she took it (literally and figuratively) as a gift from the higher powers, and used it to make herself a force of nature. And yeah, dying city or not, Selena wasn't giving her gift back.
But the big question is, is the omegahedron technology or magic? It's technology, but it gives off so much energy, that when it interacts with Selena's magic, it amplifies her powers, leading her to believe it was magical in nature.
Brother Maynard, bring forth the holy hand grenade!