No one can say with certainty how the universe will end. We must rely on a theory, a model, which in this case is very ambitious: that of the entire universe. Cosmology is the branch of physics that studies the universe as a whole.
In recent decades the “standard cosmological model” has been developed, which is based on different theories (the big bang, particle physics, Einstein’s gravity, etc.) and has had extraordinary success. It was formulated throughout the eighties and nineties and has survived an avalanche of observations. This model depends on a few parameters, which in the last 15 years have been measured with great precision.
In about 10 billion years, no more stars will be formed and in 100 billion years the existing stars will finish their fuel, cool and shut down. Everything will end in a great cold
According to this model, and the most recent values of its parameters, the universe will continue to expand at an accelerated rate. Eventually, any cosmic structure that is not gravitationally bound will disintegrate. In 100,000 million years, every galaxy that is not in the so-called local group will disappear from our horizon. We can not see them with the largest telescope imaginable. In about 10 billion years, no more stars will be formed and in 100 billion years the existing stars will finish their fuel, cool and shut down. Everything will end in a great cold.
But in this model, 75% of the content of the universe is a form of energy associated with the vacuum that does not fit well with vacuum theories as we know them. To change very little the properties of this component (retouching the model) could completely change the predictions about the end of the universe. This is why cosmologists devote many efforts to understand in detail the properties of the components of the universe.