Answer
The emitted photons are not in the visible region.
Work Step by Step
The energies of photons emitted by molecules transitioning between vibrational and rotational energy states are below the visible region of the spectrum. A typical example is shown in Example 42.3. In the cold of interstellar space, the molecules are likely to be in very low energy states, near $l=0$, emitting in the radio region of the electromagnetic spectrum.
