Recent estimates by cosmological folks suggest that around 95% of the mass in the universe is composed of dark matter and more recently minted dark energy, about which essentially nothing is known. Dark matter and dark energy don't appear to interact with our alternately glowing and dusty stuff except through gravitational means. Folks made of dark matter (if such were to exist) couldn't watch reruns of American Idol even if you forced them: they don't have any means of interacting with the broadcast signal and probably don't want to pay for cable.
For those condemned to the world of baryons and leptons, electromagnetic waves are a fact of life. In most textbooks on electromagnetic theory, you'll wade through Maxwell's equations and possibly laborious arguments on mysterious exchanges between the electric and magnetic fields launching self supporting structures with little Poynting vectors pointing out of them: all true but unnecessarily obscure. Before we go on to the mundane tasks of introducing the relevant terminology and technology of radio, let's share a little secret, implicit but not readily apparent in the standard texts, which the author has found to considerably simplify his view of electromagnetic radiation. It goes like this:
Everything radiates, but most things cancel.
To expand a bit: every object in the world that has an electric charge creates an electrostatic potential, which falls inversely as the distance. The potential sensed at some distance r corresponds to what the charged object was doing at an earlier time (r/c), because signals move at the speed of light c = 3x108 m/s. The total electric potential in the space between your nose and the pages of the book you're reading depends on the amount of charge on the fur of a cat in Bulgaria (or Wisconsin, if you happen to be in Dobrich). However, we almost never care, because electric charge comes in two flavors, positive and negative, and the amount of energy associated with an isolated charge of only one type is enormous: a microgram of hydrogen, split into its constituent protons and electrons and separated by 1 m, could support a mass of 8 million kilograms against the gravitational attraction of the entire earth. So in almost every case, adjacent to each electron with a negative charge is a proton with a positive charge, such that the two cancel, and have no net effect on your cellphone conversation.
Electric currents similarly give rise to a magnetic vector potential in the direction of the current flow, which again exists everywhere with amplitude decreasing with distance, at a correspondingly delayed time. Similar arguments show that most currents don't have any effect on distant objects: if a current is flowing in one direction, with no compensating countercurrent, charge must be accumulating somewhere, leading after a while to enormous energies (voltages). Most electric currents flow in a balanced loop: the potential from current flowing up cancels that from current flowing down, and again no net effect results on distant observers. These points are made pictorially in Figure 1, where we also introduce a bit of the mathematical terminology associated with the subject.
Figure 1. Potentials from Charges and Currents Usually Cancel
At first glance, we're left with no potentials and no waves, but, of course, this is not correct. For example, we can run an uncompensated current for a little while before charge accumulation causes too much voltage to build up and then turn it around. This uncompensated current will lead to a detectable signal at a distance. In addition, cancellation will often fail to be exact when the charges and currents are changing in time because of the slight differences in delays due to the finite size of the region over which the currents flow. For example, if in Figure 2, the loop current is suddenly turned on all around the loop at some time t = 0, the potential from the downward-flowing current arrives at r just a bit sooner than that from the upward-flowing current. Cancellation fails, and an observer sees some resulting potential: radiation has occurred.
Figure 2. Changing Currents on a Structure of Finite Size Disrupts Cancellation