On a towering volcano in Mexico, a crew of astronomers are using the Large Millimeter Telescope to try to take the first picture of a black hole. This telescope is the nerve center of a synchronized network of radio antennas as large as the Earth — the Event Horizon Telescope. If it works, the network will see details 2,000 times finer than the Hubble Space Telescope, and good enough to peer through the dust and haze that obscures the heart of our Milky Way galaxy. There, in the thick clouds of Sagittarius, astronomers have been tracking dozens of stars circling a faint radio source known as Sagittarius A*. From analyzing the orbits of these stars, they know that the mysterious source weighs as much as four million suns. And yet, it emits no visible light. If this is not a black hole, scientists don’t know what it could be. We know what a black hole should look like from Einstein’s calculations and from detailed computer simulations, like the one produced for the movie “Interstellar.” If Einstein’s predictions are right, the Event Horizon Telescope should see a tiny, circular shadow in the ring of radio emissions at the center of the galaxy. A black hole lurking like a hungry shark. In the galaxy M87, another black hole, six billion times more massive than the sun, is in a feeding frenzy. It’s shooting jets of X-rays and radio energy hundreds of thousands of light-years across intergalactic space. Astronomers think that this dragon’s breath of black holes could control the growth of the galaxies around them, shutting off the flow of gas that would otherwise be available to bake new stars. If they are right, it means the entire visible cosmos is pulsing to the invisible heartbeat of the dark side, a beat we are just beginning to see.