Does the Oort cloud exist? If so, why can't we see it?
The Oort Cloud is theorized to exist but due to many factors we can't see it.
First let's talk about what the Oort Cloud is (and what it isn't).
The first thing to know is that it is theorized that the cloud exists. While definitive proof will be difficult to come by, the theory helps explain many questions in astronomy (where do long-period comets come from and how to describe their orbits, etc)
From http://space-facts.com/oort-cloud/, it's a region of space that surrounds the solar system where ice, rock, and the occasional larger body (sometimes called Dwarf Planets) exist. The region is a spherical shell and starts about 2,000 AU (Astronomical Units) from the Sun (and to put that into perspective, 1 AU is the average distance from Earth to the Sun. Pluto is 40 AU from the Sun.) and extends well out towards the closest star (perhaps as much as 100,000AU away from the Sun or roughly 1/4 of the distance between the Sun and the closest star).
What it isn't is a cloud in the sense we think about clouds on Earth - big puffy things made up of water vapour that are easy to spot in the sky. Clouds in the astronomical sense are far less dense but are much more vast so that we see them when the light of stars and galaxies is interfered with.
So what would make the cloud hard to see?
Things that we observe in space are observable only if we can see them.
So what does it take to be able to see something? Light.
And where does the light come from? It either is produced by the object (ex. the Sun, a lamp, etc) or the light is reflected off the object (ex. the Moon).
When light is reflected, all the light that strikes the object doesn't bounce back - it gets scattered, absorbed, and the like, so only a tiny fraction of what strikes the object is reflected back (which is why the Moon, even though it can be quite bright at night, isn't anywhere near as bright as the Sun. The Moon absorbs some light and what is reflected is scattered off in all sorts of directions - only a fraction reaches Earth).
Clouds in the Earth's sky are visible because there is a strong light source (the Sun) that allows the clouds to scatter the light and yet still have plenty left for us to look up and see them. There is no light source like that for the Oort Cloud, so what light it gets, when scattered, doesn't come back towards Earth but instead flies off in random directions.
How much light does the Oort Cloud receive? Light diminishes exponentially with distance. A strong light source at distance 1 will only be
The Oort Cloud does not make its own light, so we need reflected light to see it. The Cloud is made up of small rocks and pieces of ice with huge distances between them, so most of the light that does manage to make it there passes right through and never reflects. Space rocks and ice are notoriously "dirty" - or covered in a dark coloured "space dust", so most of the light is absorbed by the rocks in the Cloud. And the distances involved are massive, so much so that to see Pluto and a few of the other large objects in the Cloud, it takes very powerful telescopes and a lot of luck in finding them.
The only other way of seeing the Cloud is with the use of a deep space probe. However, because of the vast distances, it's difficult to build a spacecraft that can make the journey. According to https://en.wikipedia.org/wiki/Oort_cloud, Voyager 1, the farthest and fastest moving probe we have and launched in 1977, will be another 300 years before it reaches the Cloud and by then will have no power to explore or send back data. Other probes also making their way out of the solar system will also run out of power long before making contact with the Oort Cloud.
The Oort cloud is a theoretical concept. It is presently beyond empirical observation.
the solar system is believed to be approximately 4.5 billion years old. This number is based on the radioactive decay of Uranium 238 to Lead. Assuming that astroids containing Uranium 238 started out with 100% Uranium the age of the rock can be calculated. The astroids commonly contain about 50% lead giving a half life of 4.5 billion.
Comets however have an observable life of 5,000 - 10,000 years.
It is assumed that the astroids and the comets were both formed at the same time when the solar system was first formed. These means that the comets must be 4.5 billion years old.
The Oort cloud is proposed as an explanation of how comets can be observed to decay and disappear in thousands of years when comets must be billions of years old according to the nebular theory of the formation of the universe.
The theory is that the comets were formed in an elliptical orbit far outside of the observable solar system. A cosmic object passing through the Oort belt can dislodge a comet causing the comet to then enter the observable solar system. As the comet made of frozen ice and dust orbits closer to the sun the comet evaporates in thousands of years. According to the theory a vast reservoir of comets exists outside of the observable solar system.
The Oort belt then is a theoretical concept that explains the contradiction between the observations of the age of the comets and the observations of the radioactivity in astroids as well as the theory of the nebular hypothesis of the formation of the solar system. Based just on the observations of the comets the solar system would be believed to be only tens of thousands of years old.