How many things can you name that are invisible? How many of these invisible things can you feel, taste, smell or hear? Do you have some other way to detect them that doesn't rely on what your normal senses tell you?
One of the things separating modern science from the science of, say, the 19th century or earlier is the sheer number of invisible things that science has uncovered and learned how to create, measure and codify. Most of the laws of nature that we know about have to do with things that are invisible, such as magnetic and gravitational fields, energy, momentum, quantum particles and their fields and of course the ubiquitous electromagnetic fields passing through your body right now. It is sometimes said that scientists only believe in things that they can see or touch. That might have been true a few hundred years ago, but it is irrelevant today. Besides, science is not a belief system.
Here is a very short list of some of the "invisible" things that we routinely work with. In principle, some of them cannot even be detected directly, no matter what our level of technological advancement may be!
Light: When you look at the page of a book, under no circumstances do you see the light as it travels from the surface of the page to your retina. When you look at a laser beam in a smoke-filled room, you can clearly see the "beam," but what you are actually seeing is the reflections of dust-particle surfaces, not the light energy in transit. You see something because a small amount of electromagnetic energy stimulates a few cells in your retina, which then signals your brain. You do not experience light energy before it activates your retina!
Crepuscular rays (Credit: PiccoloNamek/Wikimedia Commons)
Magnetic fields: We live our lives embedded in Earth's magnetic field, whose direction we can sense with a compass needle. Some animals have a "sixth sense" that lets them detect magnetic fields for navigation. The sensory organ is connected to their brains, so one wonders what they experience as they detect a magnetic field. Is it like a colorful version of human eyesight? Is it a vague "feeling"? We can see a magnetic field by using the technology of iron filings, but that is not at all the same as seeing the field itself.
Magnetic field of a bar magnet (Credit: Wikimedia Commons)
Electrons: They are the workhorses of Western civilization. All the things we do with electrons would have been considered witchcraft only 300 years ago. We know they are elementary particles far smaller than an atom, but they are not like tiny marbles and can be described equally as particles and as waves but not both at the same time. You cannot bounce a light ray off electrons to form an image of what they actually look like.
Electron waves (Credit: Don Eigler, IBM Almaden Research Center)
Gravity: We can use technology to measure its intensity as it accelerates matter, but this is not the same as rendering it "visible." The problem is that we are looking for the wrong thing. Einstein's theory of general relativity says that gravity can be "seen" only as the structure of space and time in which we operate. Space and time are gravity's field; there is nothing else!
Warped space around the galaxy cluster CL0024 (Credit: Large Synoptic Survey Telescope Consortium)
Dark matter: There is six times as much dark matter as normal matter in the universe. It only interacts with matter by virtue of its gravity and does not affect light waves directly, so it cannot be seen or photographed. It does not affect light to reflect or refract it directly except by its indirect gravitational influence. All we can do is map out its gravity to show where it is located, the way a sightless person uses tactile textures to "see" a tablecloth on a smooth wooden table.
Bullet cluster, with red representing normal matter, and blue representing dark matter (Credit: NASA HST/Chandra)
Virtual particles: The same quantum laws that define them render them impossible to directly observe. They can literally pop into existence for a brief moment, then vanish back into empty space, violating the conservation of mass for a time period regulated by Heisenberg's uncertainty principle. Yet when a photon of light is involved as a virtual particle instead of as a particle of ordinary light, you get the force we call electromagnetism. When a gluon is involved, you get the strong nuclear force that binds quarks and nuclei together. Photons can be detected as they travel freely through space carrying electromagnetic energy, but virtual photons can only be "seen" by the phenomenon we call magnetic or electrostatic forces.
Particle tracks in a bubble chamber (Credit: Brookhaven National Laboratory)
And then there are even more bizarre ideas about invisible things yet to be demonstrated or, like virtual particles, perhaps never to be observed no matter what technology we use!
Hypothetical dimensions: Some theories say space contains more than three dimensions -- and perhaps six more. Because light travels only in three dimensions and does not extend into a fourth dimension, we cannot use light to detect these dimensions, nor can we even detect them indirectly through the motions of ordinary matter also confined to three-dimensional space. These higher dimensions could be as big as our universe but are utterly invisible save for their weak gravitational influences. So much for hyperspace and Star Trek-style travel by space warp!
Solar carpet image, with magnetic fields looping back to the solar surface the way all fields are believed to "loop back" into three-dimensional space (Credit: Alan Title/NASA)
Much of modern physics has been about exploring inherently invisible things, which seem to be far more common than the obvious things we experience with our senses. Yet these invisible things follow rigorous laws that allow us to test their existence in many ways.
That is what ultimately separates the world of the imagination from the real world.