As time continued to pass, the universe kept changing in slow but important ways.
Many stars that once produced large amounts of light and energy had already finished their life cycles. Some had exploded long ago. Others had slowly cooled and become dim objects that produced very little energy.
Because of this, civilizations had to rely more and more on the energy sources that could last for the longest possible time.
Red dwarf stars became very important.
These stars burn their fuel very slowly and can continue shining for trillions of years. Many civilizations built large habitats around them so they could use their energy for extremely long periods.
In other places, scientists learned how to gather energy from unusual cosmic objects.
Some advanced systems collected energy from rotating black holes. These objects contain enormous amounts of energy that can be released slowly under the right conditions.
