HaloSat is a University of Iowa project in collaboration with NASA, who selected the project in August of 2015. The satellite is equipped with X-ray detectors in order to discover if there is a hot baryon gas halo around our galaxy in order to solve the missing baryon problem. Halosat was launched in 2018 and is currently taking data.
Baryonic matter consists of atoms that absorb and emit electromagnetic radiation.
A baryon is a subatomic particle made up of three quarks, for example a proton or neutron. Nearly everything you encounter every day is made up of baryons. A common misconception is that the missing baryon problem has to do with dark matter which is incorrect. Baryons interact with light while dark matter does not.
Because light travels with finite speed, looking further in space means looking further back in time. Peering back as far as we can to the early days of the universe, we see when it was around 400,000 years old in what is known as the Cosmic Microwave Background (CMB). Analysis of the CMB shows that the universe was uniformly filled with hot gas around 3000 Kelvin. A census of baryons in the universe today only finds about 3% of the total mass/energy of the universe, half of what is expected.
Temperature determines what type of electromagnetic radiation baryons emit:
3 to 3,000 K - radio, infrared; 3,000 to 10,000 K - visible; 10,000 to 200,000 K - ultraviolet
Because we can see the things below 200,000 K, the missing baryons are likely to be hot so we need to look at X-rays.
Evidence for an extended Galactic halo has been obtained from high-resolution absorption line spectra, but these measurements are possible only along a limited number of lines of sight. Emission lines can be measured in any direction. HaloSat will conduct a near all-sky survey of oxygen line emission to constrain the mass and spatial distribution of hot gas associated with the Milky Way. Constraints on the hot Galactic halo will help address the cosmological missing baryon problem (Wang & Yao 2007; Nicastro et al. 2012; Gupta et al. 2012; Henley & Shelton 2014).