https://www.nasa.gov/audience/forstudents/k-4/stories/nasa-knows/what-is-a-black-hole-k4.html
https://www.nationalgeographic.com/science/space/universe/black-holes/ (Links to an external site.)
http://www.astronomytrek.com/10-interesting-facts-about-black-holes/
https://science.nasa.gov/astrophysics/focus-areas/black-holes