Have you ever been to camp? What would you do if almost all of the adults left for a short trip and did not return? What if you discovered that no cell phones would work and that all communication with the outside world had disappeared? What if you discovered that a mysterious fog was covering all the world below you? How would you survive? What would you eat? What would you do as winter approached. These kids are about to find out.
They drift through space in their own orbit unrelated to the orbits of the planets, the stars, or the galaxies. Their colonies stretch for thousands of light years, and have no shape; they may be elongated, or they may be circular. They may even be cubed; or their shape may be a combination of shapes like a dry but wadded paper towel.
They can be detected, but only by using the most sophisticated of instruments, and then only by the most advanced space faring societies. Otherwise, they are invisible and undetectable.
They can be killed. Sometimes they drift into the path of a star and they burn. Sometimes they drift in the void for too many eons and they dissolve. Sometimes they drift into the path of a spear of gamma rays from an exploding star and they are sterilized. If they are detected by a space faring society that understands what they are and has the capability to do so, they are usually sterilized.
They cannot procreate on their own. They must have a host. A star with an unfortunate oxygen and water bearing planet must spin its way into their web. Even when they are fortunate to infect such a planet, it takes years for them to coalesce in the atmosphere. While they coalesce, they must disguise themselves as a pollutant. But once they have coalesced, they can become semi solid in a very short time; as short a time as twenty four hours.