A cosmology idea that I wanted to jot down. Feel free to use it.
The void of the multiverse is rife with necrotic or negative energy. We might call that entropy, heat death, friction, etc. The point is that a living creature at any point in the universe will slowly die due to exposure to necrotic energy.
Planets and asteroids are exceptions. These points of life are direct connections to the life force of the universe. In a traditional D&D sense, this may be the Positive Energy plane, but it could be any form of energy that gives life, renews, or otherwise cancels out necrotic energy.
Life evolves on planets due to the connection to this life force. Outside of the relative shelter of a planet or asteroid, the universe really is trying to kill you, but not deliberately.
I felt like thinking of cosmology this way might allow for an alternate spelljammer where ships are powered by a Spring of Life or some other gem or device that is a fist-sized planet. The small device provides air to the ship, but also propels itself by canceling the necrotic energy around it. (This would be why planets move.)
Just an idea. I may develop it more later.