<ul><li>The interstellar medium (ISM) refers to the gas, dust, and radiation filling the space between stars within a galaxy.</li><li>Hydrogen and helium gas make up about 98 percent of the ISM's mass, with heavier elements like carbon and oxygen making up the other 2 percent.</li><li>The gas molecules in interstellar space are extremely sparse, with an incredibly low density compared to Earth's atmosphere.</li><li>Dust grains in the ISM are sparse but sufficient to block visible light from stars, allowing infrared light to pass through and be observed with telescopes like the James Webb Space Telescope.</li></ul>

Scientists Say: Interstellar medium

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