Space is freezing cold, and that would naturally form a layer of Ice along the edge of the outer atmosphere to hold it in.
Space isn't 'cold'. It isn't really anything at all, as there are essentially zero molecules there to have any kind of temperature. Without a heat source, objects floating in space will eventually give off all of their internal heat via radiation, so they will become cold, but that's not because space itself is 'cold'.
The reason the tropopause is called the tropopause is because the generally progressive reduction in temperature with height that we experience in the troposphere inverts, and the air starts to get 'warmer' again with increasing altitude, becoming very 'hot' in the exosphere. I put 'warmer' and 'hot' because the concept of warmth / temperature doesn't mean what people think it means in the very thin air up there - there is very little conduction, so it wouldn't feel hot if you had your hand in it.
If you believe in either gravity or indeed UA, as per the wiki, then you wouldn't need anything to 'hold it in' at the top as it is simply the gravity or upward acceleration that causes the air to compress. However, what goes on at the edge is, of course, a different thing altogether and, if you subscribe to FET, then you would presumably take issue, at some point, with the conventional descriptions of the atmosphere's composition, given that the upper reaches of the atmosphere are beyond the altitudes given in the wiki for the moon and stars.