That's what they thought and taught when I went to college 40+ years ago...not any more.
I read awhile back they have found enough matter at the far distant outer portions of the universe that it should continue to expand forever.
First thing that pops into my poor old frazzled brain is water displacement.
Say the universe is akin to a peacefull lake. One can drop a 2 pound rock into it and watch the ripples as the rock sinks to the bottom. any small debris such as leaves or twigs floating near by are pushed away from the sight of the splash as the ripples will force them to move away from the site. when the rock hits bottom all the gunk and mud on the bottom will blast upwards float for a few minutes then sink to the bottom again.
If the lake is bottomless that rock will continue to fall, not straight down as it will drift from side to side as the layers of water change in dencity and temperature. The mass of the rock will not change but outside forces will cause it to either move downward faster or slow it down some. That rock will never attain neutral buoyancy.
As the rock falls it causes a hole in the water and sucks smaller particle in the water to fill the hole behind it while pushing against water under it.
If one were to lay under 3 feet of water and had that 2 pound rock hit you in the head, it just may kill you, but, if you are 200 feet down the velocity is slowed enough so you may just feel a slight tap as it continues to fall.
V/x /m/x/g/= e Velocity times mass times gravity equals the effect that rock will have on the surrounding matter. IE [e ] for effect.
The mass of the rock never changes but all else around it is effected to some degree. some what like a catalyst, it in itself never changes but causes changes all it touches if for only a short time.
We know there is gravity in space holding our planets in place as the speed about each other or a sun, The rogues are the comets that that have gained velocity and are going too fast to be caught, like our moon and held to the gravity of any planet. These rogues are the off shoot, like the debris of the lake bottom the rock caused to fly upward when it hit bottom. The more mass that hits bottom the larger clumps of gunk are spewed in all directions and the faster they travel.
Then Lord help us is the friction caused by any object in our waters or way out there. I remember when the Navy had to degauss some subs as their movements had caused some kind of an electronic build up because of the material they were built with.
This all comes down to displacement in our universe of the calm lake one fishes at.
Just a theory from my addled mind, good or bad, this makes sense to me, may be utter rubbish to you.