Della materia ordinaria solo l'8% e' composto da meteria luminosa. Il restante (materia oscura barionica) e' composto probabilmente sa gas (molto caldo o molto freddo) oppure da residui stellari (come nane bianche primordiali o MACHOs) La materia oscura non barionica e' composta da particelle non ancora note.
Top view of LG, projected onto plane of Milky Way What is the Local Group? The Local Group (LG) defines our local neighbourhood in the Universe consists of about 35 galaxies mostly dwarf Ellipticals (dEll) and dwarf Spheroidals (dSph) plus a few distant dwarf Irregulars (dIrr) dominated by two giant spirals: the Milky Way and M31 both giant spirals are surrounded by a group of dwarfs
LG Membership Spirals: Only three spirals in the LG: MW, M31 and M33 These spirals contribute ~90% of the light of the LG about ~90% of the mass of the LG M31 - Andromeda Milky Way M33 - Triangulum
Local group members come and go over time, with 4-5 new members per decade. LG is not complete – obvious incompleteness zone behind plane of MW. Faint dwarfs further than ~500 kpc are difficult to detect. Maybe 75% complete.
Hydra-Centaurus supercluster (contains Virgo cluster) Nearest Neighbours Perseus-Pisces supercluster We are here! Many small groups/clusters along edge of Virgo supercluster. Our nearest neighbours: Antlia-Sextans group at D LG = 1.7 Mpc; Sculptor group at D LG = 2.4 Mpc; IC342/Maffei group at D LG = 3.2 Mpc; and M81 group at D LG = 3.5 Mpc