What did two scientist discover by accident using L-band microwaves

The cosmic microwave background

Explanation:

They discovered radio noise at that wavelength.

It was constant and came from all directions.

They concluded that it was the cosmic radiation resulting from the Big Bang.

The answer is the cosmic microwave background.

The cosmic microwave background (CMB) is the thermal radiation left over from the time of recombination in Big Bang cosmology. In older literature, the CMB is also variously known as cosmic microwave background radiation (CMBR).