 Outer space, or just space, is the expanse that exists beyond the Earth and between celestial bodies. Outer space is not completely empty it is a hard vacuum containing a low density of particles, predominantly a plasma of hydrogen and helium as well as electromagnetic radiation, magnetic fields, neutrinos, dust, and cosmic rays. In the baseline temperature, as set by the background radiation from the Big Bang, is 2.7 kV-270.45 degrees C, negative 454.81 degrees F. The plasma between galaxies accounts for about half of the baryonic ordinary matter in the universe, it has a number density of less than 1 hydrogen atom per cubic meter and a temperature of millions of Kelvin's, local concentrations of this plasma have condensed into stars and galaxies. Studies indicate that 90% of the mass in most galaxies is in an unknown form, called dark matter, which interacts with other matter through gravitational but not electromagnetic forces. Observations suggest that the majority of the mass energy in the observable universe is a poorly understood vacuum energy of space, which astronomers labeled dark energy. Intergalactic space takes up most of the volume of the universe, but even galaxies and star systems consist almost entirely of empty space. Outer space does not begin at a definite altitude above the Earth's surface. However, the garment line, an altitude of 100 km62 miles above sea level, is conventionally used as the start of outer space and space treaties and for aerospace records keeping. The Framework for International Space Law was established by the Outer Space Treaty, which entered into force on October 10, 1967. This treaty precludes any claims of national sovereignty and permits all states to freely explore outer space. Despite the directing of unresolutions for the peaceful uses of outer space, anti-satellite weapons have been tested in Earth orbit. Humans began the physical exploration of space during the 20th century with the advent of high altitude balloon flights, followed by manned rocket launches. Earth orbit was first achieved by Yuri Gurgorand of the Soviet Union in 1961, and unmanned spacecraft have since reached all of the known planets in the Solar System. Due to the high cost of getting into space, manned space flight has been limited to low Earth orbit and the Moon. Outer space represents a challenging environment for human exploration because of the hazards of vacuum and radiation. Microgravity also has a negative effect on human physiology that causes both muscle atrophy and bone loss. In addition to these health and environmental issues, the economic cost of putting objects, including humans, into space is very high.