alright forgive me for saying this but who's thought that maybe nintendo/pokemon/gamefreak didn't add the light type for just this reason??? I mean it'd solve tons of problems right now by killing it off...
sadly though it still causes a few but that can all be fixed...

And another thing Dark is super weak to light no matter how small light can still be seen....
how far away is the M-44 galaxy agian???(don't qoute me there cause i dont remember its designation)
Dark = weak to Light
Light = Immune to Dark
Light cannot be weak to itself though it could be resistant/immune to itself....
Grass would be immune, Fire would be immune/resistant, Electric would be resistant, water would be resistant
Poison = Immune to light as well as not really able to do any sort of damage to light so they cancel each other there...
Steel = resistant to light (though it could do something to deflect it back at the attacking pokemon to do damage)
Bug = weak to fire, normal to light---they're like us we can live with light we'd die if the sun was too close
Ground/Rock = both resistant to light
sadly light's kinda useless cause a lot of stuff would be resistant to it...
then again maybe i'm looking at some of this wrong but you get the idea...

And as to a "ton" of poison pokemon being useless remember they made dual types for a reason...
Poison by itself may not do too well but when paired with other types it tends to start pwn teams....
Grimer is one of the OUs i believe and its a pure poison...
Go thru and look down the chart of weaknesses that poison has by itself and then compare it to the dual type poisons and see what happens
poison's strong