How we know
Light is just one stretch of the electromagnetic spectrum. NASA Science lays the whole spectrum out: radio waves, microwaves, infrared, the narrow visible band, ultraviolet, x-rays and gamma rays. Human vision is confined to wavelengths of roughly 380 to 700 nanometres, the violet-to-red slice on that chart. The CIE colorimetry standards bound visible radiation between about 360-400 nm and 760-830 nm depending on intensity, which is conventionally rounded to 380 to 700.
Measured by frequency, that visible band is about one octave. An octave is a doubling of frequency: red near 700 nanometres sits at roughly 430 terahertz, and violet near 380 nanometres at roughly 790 terahertz, almost exactly double. The full electromagnetic spectrum, from low-frequency radio up past gamma rays, spans more than eighty octaves. So the slice your eyes can register is a hairline against the whole.
Everything outside that hairline is here right now, you simply cannot see it. Your warm body radiates infrared. Wifi and radio saturate the air indoors. The sun pours out ultraviolet, x-rays stream from space and from medical machines, and the cosmic microwave background, the faint microwave afterglow of the early universe, permeates all of space and passes straight through you.
What it means
Even the sliver you do see is not raw reality. The image landing on your retina is upside down, so your brain re-orients it. Where the optic nerve leaves each eye there is a blind spot with no photoreceptors, and your brain fills it in so seamlessly you never notice the hole. And vision runs ahead of the data: the brain predicts what comes next rather than waiting for every signal to arrive.
Richard Gregory framed perception as hypothesis testing back in 1980. The neuroscientist Anil Seth calls it a controlled hallucination, the brain's best prediction of its sensory causes, checked against the senses rather than read straight off the world. You have never seen the world. You have seen a fast, useful painting of it, rendered in the dark inside your skull.
The honest caveat
"More than eighty octaves" is a standard science-communication framing and a conservative one, since the spectrum has no hard upper bound. The "faint glow of the universe" is the cosmic microwave background, which is microwave radiation rather than visible light. And the one-octave figure for vision is approximate: people vary, and under bright conditions some can perceive a little beyond 700 nanometres. None of that changes the headline. By any honest count, you are blind to almost all of reality.