The universe is expanding faster and faster widening the nearly empty space between galaxies. We know this is happening, but we don't know why. Scientists use the term dark energy to describe this accelerating expansion. It could be caused by a previously unknown force, or we may need to rethink the laws of gravity. Dark energy is a very big mystery, making up about 68% of the universe. To solve this mystery, scientists need better measurements of this expansion rate and how it has changed. Desi, the dark energy spectroscopic instrument, installed on the male telescope at Kip Peak National Observatory near Tucson, Arizona, will map 35 million galaxies in 3D by measuring their light. The range of colors or spectrum of their light will allow us to better gauge their distance and measure the rate at which they're moving away from us. This will give us new clues about dark energy. Light first reflects off of the telescope's 15-ton primary mirror. Then onto its six large lenses that ensure Desi maintains a wide, highly focused view of the sky, covering an area about 38 times larger than the full moon. These lenses are housed in a device that can be adjusted within millions of a meter to preserve the alignment of those lenses. After passing through the set of lenses, the galaxy's light reaches fiber optic cables in Desi's focal plane at the top end of the telescope. 5,000 robotic positioners, each carrying a single optical fiber, are programmed to move to pre-selected sequences of galaxy targets. This automated dance of robots allows Desi to target sequences of thousands of galaxies at a time. The optical fibers which serve as Desi's eyes send the galaxy's light down the length of the telescope to 10 spectrographs. The spectrographs separate the light from each galaxy into a spectrum of colors. These spectra allow us to gauge the galaxy's distances from Earth. Objects that are moving away appear redder because their light gets stretched to longer wavelengths. This is called redshift. Desi's spectrographs are designed to precisely measure this redshift to pinpoint a galaxy's distance and how quickly it is moving away. Taking these measurements for millions of galaxies will provide a powerful 3D map. The map will tell us which dark energy explanations fit best and whether we need to rethink our understanding of the laws of nature. Desi will function like a time machine, allowing us to see the light of distant objects up to 12 billion years into the past. This new window into the universe could help unravel the mysteries of dark energy while teaching us more about the life cycle of galaxies and the cosmic web that connects them. Thanks for watching!