Whats the black, gas like stuff around the planet in the trailer?

Well, whatever it is we have a name for it. In the newest trailer, our pilot Kallo Jath refers to it as 'The Scourge'.

I agree with boyaki. This "Scourge" has been used by the Kett to trap the Tempest; at the end of the trailer we see the ship bursting through it.

I'm pretty sure. He gets cut off at the end as the Tempest hits it, but I caught the "Scour-" part of it which can only really be Scourge with how he pronounced the part he said. 

I'm persuaded it's remnant tech, escaped from Kett control.
And it does remember me a lot of this:

Crazy theorie (not that I really believed it but still)

The 4k trailer (the one in a remnant vault), show us a vault hinting to a lot more of these vault across the system, one presumly the one they are on full of activities, soon after they are attacked by a swarm not met in trailer.

We see in the first cinematic a huge machine worm and an Emrakul one. What if this machine and the swarm are the exact same thing, just with a way higher number of bots.
We also know from the first leak that the Khett gather Remnant technology.

What if the scourge = the full armada of Remnant swarm control by the khet.
We see that the scourge as something mostly black with orange/red light depending of the scene. The same color pattern can be seen with remnant technology depending on its function Black/green, Black/blue, Black/red.

I made a huge jump in this, linking elements that already showed differences, so it probably is not the case. But the whole remnant technology is based on huge structure and maintenance robot for god-know-how-long. They having complex bots would not be too improbable to be true.

Could be an ominous cloud of death or maybe it's space dust. Who knows?

That would fit very well with Andromeda's theme.

Terraforming technology seems to be quite undeveloped in the Milk Way. But then, so is FTL travel without relays and the Arks can travel across galaxies. So I wouldn't be surprised if the Arks already posses magic terraforming technology. But if they don't, it would a huge win to aquire it.

And the trailer did give a feeling that what was once a garden world is now a planet that cannot sustain human life. So maybe the Kett terraformed it to sustain their needs? Sure, the data the Ai had is millions of year old, but that's not much in geologic terms.

I wonder if its Remnant Tech that they created and use to use

It's a symbiote. Clearly, the entire planet is turning into Venom and we won't have Spider-Man to save us this time.

It's probably some nanomachine cloud or something.