What is the heaviest known object in the universe?

Super massive black hole in NGC 4889

The answer lies about 335 million light years away from Earth in a galaxy nomenclatured as NGC 4889. Inside it in the constellation "Coma Berenices", lies a super massive black hole which is about 21 billion times that of Sun's mass. Till date, it is the heaviest object found. Its diameter roughly 6 to 17 times the size of the orbit of Neptune.

A combined infrared and visible-light image shows many of the galaxies of the Coma Cluster, including the massive elliptical galaxies NGC 4889 (bright object just left of center)

Calculations:

1 solar mass = 1.9891 × 10^30 kilograms.

Mass of our super massive black hole = 21 billion x  1.9891 × 10^30 kgs

                                                            = 4.17711 × 10^40 kgs

The density inside a black hole is so intense that even light cannot escape it.
Imagine an entire star been sucked up inside and then it turning into a size of a tennis ball ! You can estimate the density and weight of the ball right ?
This is just a case of a black hole. But super massive black holes are BIG... REAL BIG and trust me, they are not a good tourist destination for any star.

This is because a Supermassive Black Hole (SMBH), which fits the bill for being the heaviest objects, resides in the centre of most of the galaxies. Now, these galaxies aren't exactly in the neighbourhood. They lie at a distance of few million to few billion light years away from us. Hence the methods utilized to find the mass of the central SMBH will have some uncertainties. The more direct methods have smaller uncertainties, whereas larger uncertainties result due to indirect methods. An example of a direct method is measuring the period of revolution of stars lying very close to the SMBH. Other than our galaxy, Milky Way, this is possible only for very few other galaxies. The galaxies which are too far from us force us to make use of indirect methods which increase the uncertainties in the mass value of SMBH.

As of now probably the SMBH in the quasar S5 0014+81 is the heaviest known object in the universe.  It has a mass of 40 billion Suns. It is 10,000 times more massive than the SMBH at the center of the Milky Way. Wikipedia article says that the mass was derived by indirect method of calculation. Hence the actual mass may vary by a large factor. Similarly the SMBH in NGC 4889, which is on the 4th spot in the list of most massive black hole with a mass of mass of 21 billion Suns, has an uncertainty of about 15 billion solar mass.