The universe is composed of MATTER, ENERGY, SPACE, AND TIME. Beyond the universe is no matter, no energy, no space and no time (NOTHING). NOTHING is the key to the unified theory and needs to be studied! It is not composed of dark energy. That is what space is made of. The universe expands because as light or fields (e.g., gravitational, electromagnetic, etc) travels it creates space and time and is also energy and matter. Beyond where the universe's light/fields has expanded is NOTHING.
The unifying thread to Matter, Energy, Space and Time is Nothing. Matter, Energy Space and time are linked inseparable from each other.You can not have matter without space and time. You can not have space without matter, energy and time. But you can have Nothing only without matter, energy, space and time. E=MC^2
The study of Nothing needs to be pursued. It is the missing key to understanding everything.