Black holes are one of the most intriguing things space has to offer, invisible, huge, and incredibly strong. Black holes essentially cannot be seen, the only way for us to know there is a black hole is by observing other matter and with electromagnetic radiation.

source

What is a black hole?

A black hole is a region of space-time where nothing, not even light can escape due to the immense gravitational pull. Black holes can vary dramatically in size ranging from a single atom to "super-massive black holes" that are millions of solar masses in size.( A solar mass is equivalent to our sun). These black holes are theorized to form when a single black hole absorbs stars, planets and by merging with other black holes.

There is a generally accepted belief that super-massive black holes exist in the center of most, if not all galaxies and that Sagittarius A, at the core of our own Milky Way galaxy, contains a super-massive black hole of about 4.3 million solar masses.( Yes that is 4.3 million times the size of our sun)

source

The no-hair theorem states that, once it achieves a stable condition after formation, a black hole has only three independent physical properties: mass, charge, and angular momentum.
When a black hole swallows an object or something "falls into" a black hole all information about said object is lost completely as it becomes evenly spread along the event horizon, this is also the part of the black hole from which nothing is able to escape the gravitational pull.

The event horizon is considered the defining feature of a black hole, after this point nothing, not even light, are able to move outwards and away from the center of the black hole. "The event horizon is referred to as such because if an event occurs within the boundary, information from that event cannot reach an outside observer, making it impossible to determine if such an event occurred"

source

Known black holes

Alright im not actually going to list the known black holes as most of them are just a series of numbers or letters or combination of both e.g "NGC 1313 X-1".
As mentioned earlier it is a generally accepted theory that there is a super-massive black hole within the center of most galaxies and whilst they can vary dramatically in size most are many millions of solar masses.

The largest

Ton 618 - The largest known black hole, around 66 billion solar masses

The closest

A0620-00/V616 Mon, approximately 3000 light years away from earth

The smallest

GRO J0422+32 though there is some debate as to whether its indeed a black hole.

source

Evaporation

Whilst black holes emit no observable light radiation or anything else for that matter Stephen Hawking has theorized that they do release small amounts of thermal radiation using quantum field theory.
If this theory is correct it is expected that over time they shrink due to loss of mass. The temperature of this thermal spectrum is proportional to the surface gravity of the black hole, meaning that larger black holes would emit less radiation than smaller black holes.

Essentially it means large black holes would not shrink at the same pace as they grow and therefore would not "evaporate" whereas small black holes would indeed be able to evaporate into nothing. Though for a black hole to be small enough to evaporate it would need to be around the same mass as our moon which would mean its diameter is less than 1/10th of a millimeter... Can you picture that though? Something with the mass of our moon compacted down to less than a millimeter.
A black hole with the mass of a car would have a diameter of about 10−24 m and take a nanosecond to evaporate at which time it would emit luminosity more than 200x that of our sun.

source

Black holes would have to be one of the most intriguing and interesting thing the universe has to offer, while still very little is known about them we love to explore,observe and theorize on them. They have been made popular because of their inherent destructive properties and the fact people believe they may be a key to time travel, though so far any time travel theory ( at least to the past or distant future) has been written off.
Space has so many unknowns and since black holes are almost unobservable we know very little about them. Who knows, maybe earth will one day be swallowed by a black hole, in fact maybe one day the entire known universe will be one giant black hole, i suppose its entirely possible :P

If you like what i post please upvote follow or even resteem

Steem on Steemians

Resources
https://en.wikipedia.org/wiki/List_of_black_holes
https://en.wikipedia.org/wiki/Sagittarius_A*
https://www.nasa.gov/audience/forstudents/k-4/stories/nasa-knows/what-is-a-black-hole-k4.html
https://en.wikipedia.org/wiki/Black_hole#Physical_properties
https://science.nasa.gov/astrophysics/focus-areas/black-holes