Venus is the second planet from the sun, and the closest on average to Earth. It has historically been called “Earth’s twin” due to their similar size, composition, and relative distance from the Sun. Despite this, Venus also has an atmosphere made primarily of carbon dioxide with clouds of sulphuric acid that is dense enough to produce a pressure of 93 bars, roughly equivalent to the pressure under 900 meters of water on Earth, along with an average surface temperature of 462 degrees celsius, making it the hottest planet in the solar system. All of these factors combined have made Venus an incredibly hostile place to both human exploration and habitation, and as a result Venus remains uninhabited even while both Lunar and Martian populations continue expanding.
This is not to say that Venus has been entirely ignored, however. While the surface at present remains largely off-limits to manned exploration and permanent settlement, robotic probes have carried out continued exploration of the Venusian surface. At the same time, Venus has also provided an important source of carbon dioxide as a greenhouse gas for the terraformation of Mars, resulting in a significant human presence in Venus’ sphere rather than directly on its surface.
The primary industries of Venus have taken the form of floating collection plants in the upper atmosphere of the planet, where the pressure and temperature are within human tolerances. CO2 containers can then be elevated to orbit, where the many Venus-Mars transfer windows can then transfer the gas directly toward Mars for use in warming the surface. The degree of activity around Venus have also resulted in a proposal to make use of either Main Belt asteroids or Near-Earth asteroids to create an artificial moon (similar to Phobos or Deimos around Mars) that can act as a central base for Venus sphere operations. The proposal was realized in 2097 when 524522-Cupid, a quasi-satellite of Venus, was successfully brought into Venusian orbit. Cupid has been the site of "Cupid Base", a hub for ships entering and exiting Venus' sphere, since 2104.
Unlike Mars, terraformation and colonization of Venus is a much more challenging prospect. Whereas Mars has a thin atmosphere and low gravity, making transfers to and from the surface relatively easy, Venus’s dense atmosphere and gravity of about 0.91 that of Earth makes surface to orbit transfers more difficult than even on Earth, without taking into account the sulfuric acid that eats away at most metals and plastics. Terraformation would require extensive atmospheric engineering and work on the surface to deal with the sulfuric compounds that have formed over the years. As such, even while the populations of Mars and the Moon have expanded into the millions, Venus’ will likely remain at 0 for possibly centuries to come.