They cannot look in deep.
Interstellar gas consists of denser, cooler clouds embedded in hotter intercloud gas.
The interstellar medium is detectable because it emits, absorbs, and reflects radiation. Nevertheless, interstellar space is not entirely empty. Let me tell you. By contrast, the lowest density regions of interstellar space contains ionized gas with a density of approximately 10 -4 molecules per cubic centimeter. 3 made of molecules 2.