How can the universe keep expanding?
The force of gravity is not sufficient to bring the matter and energy back together.
Newtons first law of motion is that matter will continue in the same direction and speed unless acted upon by an outside force,
The debate has been if there was enough matter to create a gravitational force strong enough to reverse the momentum observed by the red shift. The hypothesis was that there would be enough matter including dark matter to cause the universe to come back together in a Big Crush. This hypothesis has been proven false.
The most recent empirical evidence is that the rate of expansion of the universe is actually increasing, not slowly down. This indicates that the universe is an open system, not a closed system. This means that the universe will continue to expand until the heat death of the universe results.