Based on current evidence, this is how we expect our universe to evolve. The amount of matter needed to exactly close the present Universe is usually described as the critical mass. Here is where dark matter becomes important. Remember that dark matter is mass whose presence we infer from its gravitational effects but which gives off no radiation nor other sign of its presence. Matter becomes thinner and thinner, the Universe becomes colder and colder.
New stars become harder and harder to form, so eventually all the stars left are low-mass stars.
One by one they die. Emission nebulae and reflection nebulae go dark. Finally there is nothing left but dark gas and dust and the cold, cold background radiation of the universe.