I have been pondering this question. A sphere of xenon, dense enough for animals to be buoyant and large enough to have gravity? You would need calculus to model gas density at any depth and ultimately you have made a gas giant. As regards cruising around in a gas giant, XKCD has had the last word. A nebula? Too nondense. Some space vortex! I think a vortex would hold together in space, but it would slow rapidly from frictional energy losses. Powered by solar wind?
The OP wants earth like atmosphere in a zero-gravity environment. Like the space station. The space station contains its atmosphere in a hollow shell. That is the solution. Your world is inside a hollow planet, and inside the shell of this planet there is no gravity.
A hollow symmetrical sphere exerts no gravity on objects within, regardless of how much gravity an object outside on the surface might experience. https://en.wikipedia.org/wiki/Shell_theorem
This concept got a little bit of attention on WB stack here. Does the thickness and diameter of a hollow earth give an affect to the overall gravity inside?
but I think lurch's explanation of how this works is excellent. I excerpt it from the Halfbakery.
But the object inside a hollow sphere experiencing no gravity - that
does not seem right. An object in the exact center of Pellucidar would
experience no gravity as the pull from all sides would cancel out,
just as the object falling thru a tunnel piercing the earth will
oscillate back and forth until coming to rest at the center.
Would not an offset object (here the interior orbiting device as > >proposed) experience more pull from the nearer wall than the
farther (inverse square and all)? — bungston, Feb 29 2012
The object experiences more pull from a given mass on the near side, but >more of the mass of the sphere is on the far side.
Think of four equal masses, spaced 90 degrees around a circle. Put, on a line between two of them, an object, closer to one than the
other. That nearer body excerts more pull than the farther. However,
the two to the sides cancel out each others lateral pull, but also add
resultant force away from the near object. Once you extend this to a
sphere (and do a bit of calculus) all the resultant forces cancel out.
— MechE, Feb 29 2012
Your hunch about inverse square distance is correct, but it has another
effect you've neglected.
OK, you are about to be placed inside the shell of a hollow planet. You > will not be in the center, but far off to one side. You
are given a laser pointer - not some puny dollar-store thing, but a
megawatt-class laser with beams coming out both ends. Be careful
Now, taking your Darth Maul laser-pointer, you point it in any direction >you please, and move it around to draw a shape. You will
notice that if you point one end in a direction where the shell is
close, then the other end is directed at a spot far away.
Consequently, when you draw your shape, the close end draws a small
shape; the far end draws a big shape - but they subtend identical
If you now calculate the gravitation exerted on you by the two indicated >areas, the close (small) one exerts more force on you per
unit mass than the far (big) one - because of the inverse square law
as you had presumed. However, the "per unit mass" caveat, when applied
to the smallness/bigness of the two opposed figures, cancels
It works for any opposed pair of shapes you can draw, from any position >inside the shell, as long as the shell is uniform.
— lurch, Feb 29 2012
Consider such a planet with a colony on the exterior surface. There is
a door in the floor of one building. On opening it we see people
floating around down there; they are on the interior side.I wonder
what the experience would be of coming up through that door from the
weightless side. — bungston, Mar 16 2012
//coming up through that door from the weightless side.// huh... cool. >I'd never thought of it quite that way - it would feel
like an anti-gravity repulsion field was trying to keep you away from
the door. (Actually gravity toward everywhere except the non-existent
mass in the shaft to the doorway, so the imbalance would seem like a
"Shut the door so I can get out!" — lurch, Mar 16 2012
This can be your world: a hollow planet inside of which there is trapped atmosphere but no gravity. The cool thing - as noted above, if you open a door to the outside, Pellucidar will try to keep its atmosphere inside.