I have heard the term "cosmic string" and am not sure I
understand the definition. Is it left over matter?
Cosmic strings are thin strands of ultrahigh density matter
that are predicted by some theories to have been left over
from an extremely early era of the universe. Cosmic strings
would have a width that is far less than an atomic nucleus,
and a mass of about 10 million billion tons per centimeter. A
kilometer of cosmic string material would weight as much as
the Earth! They would make closed loops or stretch across the
universe and perhaps have an infinite length.
Some cosmologists have speculated that cosmic strings could
be the dark matter, or seeds for cosmic structure but this
idea turned out to be incompatible with observations. Even
so, cosmic strings could still exist, and might eventually be
detectable through their gravitational effects.
Cosmic strings are not to be confused with superstrings, the
tiny subatomic loops of matter that according to superstring
theory are the fundamental building blocks of all particles.
References: J. Gott, Time Travel in Einstein's universe
(2001, Houghton-Mifflin, New York).
M. Rees, Just Six Numbers (2000, Basic Books, New York).