According to this theory, the resource consumers in the universe are just as vast as the universe itself. At the end of the series the final resource constraint

According to this theory, the resource consumers in the universe are just as vast as the universe itself. At the end of the series the final resource constraint they deal with is running out of physical matter in the universe. Once the civilizations depicted in the book are done mastering survival in this universe, they all turn their attention to surviving the death of the universe. Stability simply moves the survival goalposts, and “we could probably last a few billion years” (or rephrased, “we will probably perish in a few billion years”) becomes the new existential crisis to solve.