NASA plans to launch on Monday a sounding rocket to study the darks voids in between the stars and galaxies that fill the night sky.The cold, diffuse gas between galaxies -- called the intergalactic medium, or IGM for short -- hardly emits any light.To shed light on the nature of the IGM, the sounding rocket will be equipped with special ultraviolet optics, NASA said on Saturday.The experiment will launch from the White Sands Missile Range Las Cruces in New Mexico. The Dual-channel Extreme Ultraviolet Continuum Experiment, or DEUCE for short, plans to measure starlight from a pair of nearby hot stars in the constellation Canis Major, aiming to help researchers understand how the IGM got to its current state, NASA said.Scientists know that the IGM, which is mostly hydrogen, has been blasted with high-energy radiation, causing the electrons to break apart from their atoms -- a process known as ionisation. Many think intense ultraviolet starlight from star-forming galaxies is responsible for ionising the universe, but not all agree this is the sole cause. Since Earth's atmosphere blocks ultraviolet light, it is impossible to study this type of radiation from the ground. Instead, scientists must capture this light from above the atmosphere and sounding rockets -- which provide an inexpensive alternative to space telescopes -- are a practical option."DEUCE is about being able to better understand if and how star-forming galaxies ionized the early universe," said one of the researchers involved in the project Nicholas Erickson from University of Colorado Boulder. "This ionising light has never been measured accurately in hot stars, and DEUCE will make the first calibrated measurement of it, telling us the contribution stars could have had to helping ionise the universe," Erickson said.