yeah it’s mostly hydrogen but it wont burn cuz there’s no oxygen in space lol. the 10 million degrees isn’t a typo - it’s actually a super-hot plasma at that point, not really a gas anymore. our sun’s core is around 15 million degrees for comparision.

To your first question:

Yes, it is hydrogen, or at least, that is what they measured, the hydrogen component, and hydrogen is… something like 97% or 98% of the normal mass (ordinary matter) in the universe.

No, a spark would not light a cloud of hydrogen, in space, on fire.

That is because hydrogen requires oxygen to combust.

Space is not our atmosphere, and there is very, very little ambient oxygen in space.

…

Second question:

So that given temp of ~10 million degrees… is in Kelvin. It is not an error.

Our Sun varies from about 15 million K in its inner core, to about 5.5k K on its surface.

The entire way they are measuring this giant cloud tendril is very much reliant on X-Ray band emissions.

If a cloud of Hydrogen is approximately 10 million Kelvin, it will be a highly energetic plasma, with electrons and protons smashing into each other and emitting x-rays.

So… its more like a plasma cloud than a gas cloud.

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Now this part is where we start to exceed my grasp on astrophysics, but uh…

I think, basically, what is going on here is that… these filaments of super hot hydrogen plasma, which connect galaxies, but have no stars in them…

This is left over super high energy hydrogen plasma from the earlier universe, that did not manage to become dense enough to form stars and galaxies… and basically got accelerated by star formation and eventually galaxies rotating… but didn’t get included in said stars and galaxies, sort of got thrown out, and then just got stretched out further and further as the universe aged, and galaxies spread apart from each other.

So it is both very non dense, and also very high energy.

…

If you’ve ever played some kind of spaceflight type video game, and gone into a ‘gas’ cloud, except this one is making all your radiation alarms go crazy, eats through your hull, nullifies your shield, etc?

Yeah, that’s maybe a more relatable way of grasping what this stuff is?

More realistically, it would likely be so sparse, so non dense, you wouldn’t be able to even visually see it…

Though, the constant bursts of X Rays may basically be temporarily blinding you or appearing as extremely bright dots and specs in a human’s optical perception, sort of like how a digital camera freaks out when something is so visually bright it breaks its output.

And that would be as you are approaching it. If you were significantly in the plasma cloud… you would probably just melt/disintegrate if you were close enough to it, or inside it, for too long… it is 10 million degrees K, after all.