Yes, photons from the cosmic microwave background (CMB) radiation can indeed enter our eyes, even though we cannot see them directly. The CMB radiation is a remnant of the early universe, which is composed of photons that have been traveling through space since the time when the universe became transparent, about 380,000 years after the Big Bang.
These CMB photons have wavelengths in the microwave range, which is much longer than the wavelengths of visible light. Our eyes are specifically sensitive to a narrow range of wavelengths within the electromagnetic spectrum known as the visible spectrum. This spectrum includes the colors we perceive, ranging from red to violet.
Since the CMB photons have much longer wavelengths than those within the visible spectrum, they fall outside the range of wavelengths that our eyes are capable of detecting. Therefore, while CMB photons can enter our eyes, our visual system is not equipped to directly perceive or interpret them as light. We require instruments such as microwave detectors or telescopes that are specifically designed to detect and measure microwave radiation to study and analyze the cosmic microwave background.