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At the beginning of the 20th century, Albert Einstein replaced the aether theory by relativity, but a 21st-century aether is still puzzling physicists today. This modern aether is the quantum vacuum. The aether was thought to be an all-penetrating mysterious substance that carries light through space like air carries sound. Take away all light, and the aether would still be there, defining a universal frame of reference. Now, according to quantum field theory, the state of absolute darkness, the vacuum state, is still a physical state filling space throughout, similar to the aether. There is an important difference though: one does not notice motion at uniform speed relative to the quantum vacuum, but, as we describe in this lecture, during acceleration the vacuum glows, although slightly, causing friction. Furthermore, as Stephen Hawking predicted in 1974, the quantum vacuum should also cause black holes to evaporate, because at the event horizon particles are created from nothing, at the expense of the black hole's mass. In this lecture we also describe the creation of radiation at horizons. Finally, we will turn to forces of the quantum vacuum, in particular the Casimir force. These examples show that the physics of the quantum vacuum is real, although we do not fully understand it.