Well, here goes nothing:
Dark Pokemon are among the most unfairly maligned Pokemon, in part due to mankind's natural diurnal tendencies. In reality, what is branded as armies of demons and blackness is actually one of the most diverse groups of Pokemon, and some might even offer us clues about parts of Pokemon evolution as a whole.
Historically, it was considered that most Dark Pokemon were nocturnal. Studies have shown, however, that species like Hydreigon, Murkrow/Hunchkrow and Absol are actually cathemeral, if not outrightly diurnal in at least some population complexes. Needless to say, though, they are more effective and powerful at times of low light. The main speciality of Dark types seems to be in direct offensive, having relatively low physical defense and stamina in most cases. This seems to be related to the nature of their abilities, which are quite energy expensive.
One characteristic many people see in Dark types as worthy of extermination is their pragmatism, being accused of "fighting dirty". The reason for this is related to the above: the lack of stamina and defense forces most Dark types to resrot to faster means of self-defense or hunting. In addition, several species are solitary, which reduces social instincts. The fact that many steal is also not good for their reputation.
Dark types are notable absent from Kanto. Subfossil remains show they were present there as recently as a few hundred years ago, but became locally extinct due to ruthless extermination. Since then, programs of reintroduction are being highly debated.
Dark types usually resort to physical abilities, but when pressed they are capable of utilising what is often reffered as "Necrophotokinesis" (literally, "using black light"), informally reffered as "Umbrakinesis". The Pokemon basically manipulates a strange form of energy, very similar to light, but resembling physical darkness, which they draw from the sorrounding environment. It has been speculated that this is actually dark matter, though no conclusive studies have been made on the matter. Studies on Hunchkrows seem to suggest that this evolved from actual photokinesis, though why and how it became this is unknown. At any rate, the Pokemon can condensate it into forms apropriate for offense, though otherwise it is naturally no difference from light in terms of physical effects.
Necrophotokinesis/Umbrakinesis has other benefits besides direct attacks. Even miniscule amounts of this radiance seem to render psychic attacks highly impotent, likely via disruption of electromagnetic waves (his has in turn evolved into highly efficient mind manipulation in Darkrai). It can also create illusions, allowing Dark Pokemon to steal efficiently. In turn, however, much like physical light, it attracts insects, and Bug types in particular are highly resistent.
Dark types have also high resistance to Umbrate, though this doesn't seem to be related to Necrophotokinesis, though it may be related to how they manage to manipulate it. Steel types, with their capacity of reflecting physical light, can also reflect the Dark type radiance.
The emphasis on quick, pragmatic attacks puts Dark types highly at odds with the more ascetic Fighting types.
Pure Dark types seem to be mostly mammals, Darkrai excepted (maybe?), suggesting that an endothermic metabolism is necessary for Necrophotokinesis. Birds and other endothermic creatures are also Dark, though considerably rarer.
Some Dark-type have waving, smoke-like appendages, such as Darkrai, which might facilitate their abilities.
Most Dark types are highly opportunistic omnivores, with very few true carnivores or herbivores, as their high metabolisms make pickiness difficult. Most can make a living in urban environments, being efficient garbage scavengers and experts at stealing, though some, like Absol, preffer isolated environments, away from civilisation.
Most Dark types, being mammals or birds, have extensive and dedicated parental care, often longer than in closely related Pokemon from other types. More phylogenetically confusing creatures, like Hydreigon, leave their young to fend for themselves.
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