In simple words, Dyson sphere is a proposed man-made structure that encircles Sun to capture and use most of its energy. This energy can then be somehow transferred to Earth for meeting our energy needs.
It is named after Dr. Freeman J. Dyson, who proposed it first in 1960 in his famous research paper, that can be found here.
Why we need a Dyson sphere?
We are running out of fuels options on Earth. Renewable energy, such as wind, water, etc are good alternatives, though they don't generate as much output as conventional energy sources. Solar energy is a very good option to fulfill our needs but only a tiny fraction of sun's energy reaches Earth. Thus, it can be a very good option to build a megastructure around or close to Sun that can capture most of its energy for our use. This structure can be lined with mirrors on the inside(millions of solar panels). This can provide us over a trillion times more energy than we can produce or capture on Earth.
Can we build a Dyson sphere?
In 2 words, currently no. We don't have enough resources or technological advancement for us to develop such a huge device. This proposed structure is for the future, when humanity is much more technologically capable and strong to even start thinking about such investments. According to Dyson, "One should expect that, within a few thousand years of its entering the stage of industrial development, any intelligent species should be found occupying an artificial biosphere which completely surrounds its parent star. "
Sun's diameter is around 109 times the diameter of Earth. It will take most of our resources, plus more, to build something that. Then there is the challenge of transporting such a structure to sun, and deploying it, which will present different types of challenges. So, realistically speaking, at the moment we are not capable enough to build such a structure and send it to Sun.
Types of Dyson sphere
Though, very challenging, there are lot of ways we can build such a structure, some of which can be possible in very near future as well!
It is basically a metal sphere that covers the sun. It is technically very impractical, since building such a structure will, firstly, use most of our resources. Transportation is also an issue. But other major concerns include very high tensile strength (or how strong the structure is, so it doesn't break), and sun's gravitational pull, which can make parts of the structure very unstable.
Dyson mirror swarms
Now this is much viable than previous method. It is a swarm of mirrors or solar panels floating around Sun to capture its energy. You can imagine it as bees surrounding the sun, where each bee is a solar panel. This is more practical and can be even possible in next few decades, where a basic version of Dyson sphere can be launched. The resources can also be mined from nearby planet, like Mercury, and robotically sent to the Sun to join the sphere.
Do Dyson spheres exist in universe?
We don't know for sure, but there has been some instances where signals from space indicated towards the presence of Dyson sphere. One very famous incidence was discovering of the star KIC 8462852 in Cygnus constellation, 1600 light years away. The light from the star was captured 'flickering' or dimming, hinting towards an alien megastructure going around the star. This news was very popular in media in 2016, but now scientists believe that there can be a non-alien explanation for such a behavior.
Since they emit infrared radiation, Dyson spheres are also a candidate for finding alien life forms through this technological signature. Till date, no such structure is found. Now, a team in the Gaia mission in Europe specifically focuses on finding Dyson spheres through different methods than the conventional one of 'dimming of stars'. They have already found up to 800 candidate stars that may have Dyson spheres around it!
Though not currently found in the universe, they are found everywhere in fiction! Olaf Stapledon's novel "Star Maker" from 1937, and Larry Niven's novel "Ringworld" from 1970 mention this kind of structure. Even a 1992 Star Trek: The Next Generation episode showed a star with a rigid spherical shell around it.
Dyson spheres can be a viable energy source for us in near future, when we have enough resources and advanced technology. Till then, we can definitely think more in this direction and perfect this amazing idea!