Ultima Thule is an ancient phrase which means "borders of the known world". On a map from 1539 its represented as a place where monsters lurk.
In this more scientific age, it refers to an 'object' which lurks far in the distant reaches of the solar system, 46 times farther from the Sun than Earth. Its formal name is "(486958) 2014 MU69" We know so little about this thing, that it may not be a single thing at all - it could well be several things close together. Our scientific knowledge of it comes entirely from a mix of light and darkness.
On the side of light, we have a total of 144 photons which traveled to us the 380 light-minutes distances before colliding with the sensors on the Hubble space telescope last year.
On the side of darkness, scientists have traveled to remote points on the earth to witness the microscopic shadow Ultima Thule casts on our planet as it eclipses even more distant stars. As a few photons which should arrive to us dont, we gain information on how big the object is. Its probably 30km across.
Ultima Thule is so dark and cold that scientists think it has existed since the formation of the solar system and hasn't had any interaction with anything else since. Totally unchanging, dead and silent, for nearly 5 billion years.
Why is this interesting? Well, on January 1st 2019, a spacecraft called New Horizons. Its left earth in 2006 and has been moving at 50,000 km/h since. You may remember that in 2015 it buzzed past Pluto and took an amazing series of photos.
Ultima Thule is next. We have no idea what New Horizons will show us.