In 1992 Alex Gurevich proposed that lightning is the spawn of interstellar radiation. The theory was widely dismissed by the scientific community, but in the time since it has come more and more into favor. Partly because there has been no suitable scientific theory, and partly because new observations fit correctly into the theory.
Various agencies studying lighting discovered in 2002 that lightning produces a large pulse of x and gamma rays. This was a surprise since conventional wisdom said that the atmosphere is too dense to allow for electrons to accelerate to speeds high enough to create those high-energy emissions. It was perplexing, but the much older proposition of Runaway Breakdown explained it eloquently.
The high energy Cosmic Rays smack into air or water molecules in the conundrum, and cause the molecule to release a high energy electron. That electron has a reach and power usually unavailable in the atmosphere, and that electron is prone to hit another molecule which will release one or more electrons in turn. This cascading breakdown is somewhat analogous to a nuclear reaction in an open environment. but where a nuclear core must be cooled, inside a thunderhead the energy is dispersed when there are enough long-trajectory electrons created, and they conglomerate into a bolt of conductive plasma--lightning.
There are still some difficulties with the Runaway Breakdown theory, namely that the events described therein are exceedingly difficult to observe since storms rarely make appointments to appear at a designated time and place for observers. This isn't to say it's not provable, but like may things, it will take time. As Benjamin Franklin, the father of meteorology and electrodynamics said, "He that can have patience can have what he will."