Colonel Mark Medula is an astronaut for U.S.I (United Space Industries) and an officer of the R.A.A.F. While returning from a manned mission to Mars, he encounters an alien war lord from the planet Callie who had killed the crew manning the international space station Threshold. This alien ( Cerellus) is hell bent on the conquest of earth so his people can have a world on which to survive. Mark and Cerellus battle and kill each other and both souls are given another chance at life by sharing the same body.
Dark Nebula receives his powers from the two souls occupy his now dead body. Cerellus an alien warlord from the planet Callie and Mark himself. From Cerellus he receives psionic abilities (telepathy, telekinesis etc), from Mark he receives the dark fire (a powerful mystical force. Though Mark was unaware of it at the time he is decended from a long line of witches) There is a constant battle for control of the host body a battle of wills that only ends when Cerellus' people come to find him. Before then, Cerellus constantly attempted to take over Mark's body at every opportunity.
When Mark and Cerellus are separated on Cerellus' home world. Mark faces judgement by the entire race, with nothing but his Dark fire to rely on he cuts loose destroying the entire populous and in the process elevating them to the next stage of evolution as immortal incorporeal beings. After this event Cerellus continues to share the body with Mark giving him access to his powers without fighting him for control. It's his way of thanking him for helping his race survive ( something that would not have happened without evolving)
When Mark returns from the Callien planet, 18 months have passed. Dark Nebula's enemy Grandstander takes advantage of his knowledge of DN to imitate him and frame him for crimes and treason. As such when Mark returns ( after what he thinks is only three weeks) his reputation is ruined. His wife believes he abandoned her and he is wanted by his own government and hunted down by the Southern Squadron. He is cleared only because once the Squadron capture him ( having not released that fact ) the DN amazingly still commits crimes while he is in custody. Once they realise Mark is innocent, both Dark Nebula and the Southern Squadron team up to take care of Grandstander.
Telekinesis (powerful enough to hold a city bridge together fully laden with traffic)
Dark Fire ( a powerful mystic blast power strong enough to destroy the entire over populated populous of a highly advanced civilization. Also powerful enough to hold back the entire hordes of hell)
Super strength (though it's not known if Mark uses is other abilities to achieve this effect) he has been seen smashing cement and bending steel.
Though it's never been clearly identified or quantified there is reference to Dark Nebula possibly being indestructible to some degree. When the Grandstander blasts and apparently kills him in his first solo issue, Mark returns, leading to Grandstander saying "You're dead! I killed you!". Mark replies "It's pretty hard to kill someone who's already dead". This is the only time that any reference to such an ability is made. Possibly due to the fact that the adversaries he faces from then on are far more formidable and therefore capable of harming him. Adversaries such as "Chaos" a mystical force of devastation, the entire forces of hell, an entire advanced alien super powered race, forces from all over the time stream just to name a few.
Sometimes, Cerellus's personality will take control of Dark Nebula, causing him to act more violent and rude.
Ann Medula - Wife
Josh Medula - son
Unlike most superheroes, Dark Nebula swears when he is in trouble, usually saying "Shit!". Dark Nebula can also be pretty blunt, such as when he first met Grandstander (who at the time was believed to be a superhero), and Mark thought to himself "Geez, he's pretty up himself".
You can read some of the Dark Nebula's adventures online at http://www.webcomicsnation.com/darknebula/