"New Horizons" is a NASA mission was launched in 2012 to explore far regions of the Solar System. The first important "stopover" of this spacecraft was a dwarf planet Pluto and its moon called Charon. Thanks to this "visit", now we have many high resolution images of these two objects and know far more about them. For example, it is known now that under Pluto's nitrogen and methane crust there is an ocean of liquid water.
Having sent the invaluable data back to Earth, "New Horizons" hibernated and resumed its way farther into Kuiper Belt. It would be more appropriate to call it a donut instead of belt, actually. Its objects, known as Kuiper Belt objects (KBOs), orbit around the Sun at a distance between 35 and 55 a.u. (1 astronomical unit = distance between the Earth and the Sun). Some of them are large (1200-2700 km in diameter) and massive: Eris, Pluto, 2007 OR10, Makemake, Haumea and Quaoar. Others are pieces of primordial rock and metal, and the rest are large chunks of ice made of water, ammonia and methane. Many of its objects are in so-called resonance with Neptune. For example, Pluto orbits the Sun twice in the period of time it takes Neptune to complete its orbit 3 times.
It is hard to believe that the Kuiper Belt has been officially discovered only 25 years ago. Astrophysicists suspected its existence, as it would have explained the short-period comets, such as Halley's Comet: passing close to the Sun they lose a lot of their material, so they need to replenish it somewhere regularly. The period length excludes a possibility of a comet returning to the Oorth's Cloud (a hypothetical sphere of comets surrounding the Solar System at 50 000 a.u. distance, whose existence we haven't confirmed by observations yet), so it has to collect the matter from somewhere much closer. In 1992, David Jewitt and Jane Luu announced the "Discovery of the candidate Kuiper belt object". Futher observations made evidence for the Kuiper Belt existence only stronger, and nowadays we know that the Belt consists of hundreds of thousands objects ranging in size and composition.
Given the timing, Voyagers and Pioneers have passed through the Kuiper's Belt without registering any of its content, because back in those days we didn't really know that there was anything worth to look at.