The research by the School of Physics and Astronomy at Tel Aviv University has been called a major new finding about the origins of the universe.
The black holes also imprinted a clear signature in radio waves that astronomers can now search for, the researchers said.
"One of the exciting frontiers in astronomy is the era of the formation of the first stars," Tel Aviv study author Rennan Barkana said. "Since the universe was filled with hydrogen atoms at that time, the most promising method for observing the epoch of the first stars is by measuring the emission of hydrogen using radio waves."
Cosmic heating may offer a way to directly investigate the earliest black holes, the researchers said, since it was likely driven by star systems called "black-hole binaries," pairs of stars in which the larger star ended its life with a supernova explosion that left a black-hole remnant in its place.
Gas from the companion star pulled into the black hole then created high-energy X-ray radiation, believed to have reheated the cosmic gas after it had cooled down as a result of the original cosmic expansion, they said.
The discovery in the new research is the delay of this heating, the researchers said.
"It results in a new prediction of an early time (when the universe was only 400 million years old) at which the sky was uniformly filled with radio waves emitted by the hydrogen gas," Barkana said.