<P> with λ _̄ ≈ 0.035 (\ displaystyle (\ bar (\ lambda)) \ approx 0.035) and σ l n λ ≈ 0.5 (\ displaystyle \ sigma _ (ln \ lambda) \ approx 0.5). At all halo masses, there is a marked tendency for halos with higher spin to be in denser regions and thus to be more strongly clustered . </P> <P> The nature of dark matter in the galactic halos of spiral galaxies is still undetermined, but there are two popular theories: either the halo is composed of weakly interacting elementary particles known as WIMPs, or it is composed of a number of small, dark bodies known as MACHOs . MACHOs, an acronym for massive compact halo objects, would be composed of "ordinary" matter that simply does not emit easily detectable radiation . The reliance of modern astronomy on detecting electromagnetic radiation would render dim, but massive objects, nearly undetectable . A wide range of possible MACHO candidates have been suggested, encompassing everything from black holes to very dim dwarf stars . Despite being too dim to detect directly via electromagnetic telescopes, MACHOs would necessarily interact gravitationally, as described by general relativity . The preferred method for searching for MACHOs in the halo of our own galaxy has been to look for microlensing events . Gravitational microlensing occurs when two stars fall on a common line of sight, rendering the far star obscured by the near one . However, as light from the far star passes through the gravitational well of the near star the light bends, creating an Einstein Halo . In a microlensing event, the halo is so small that it is optically indistinguishable from the star . The overall effect is to simply make the star appear brighter . The EROS and MACHO projects search of MACHOs in the halo by observing stars in the small and large Magellanic clouds . If a MACHO existed in the halo along the line of sight of stars in the cloud, the lensing would brighten it from that orientation as opposed to others . The magnitude and number (or lack of) lensing events can be used to place bounds on the masses of any MACHOs which might be in the halo . The two projects initially were able to place a very strict limit on MACHOS in the range of 3.5 ⋅ 10 − 7 M ⊙ <m <4.5 ⋅ 10 − 5 M ⊙, (\ displaystyle 3.5 \ cdot 10 ^ (- 7) M_ (\ odot) <m <4.5 \ cdot 10 ^ (- 5) M_ (\ odot),) concluding that such light mass MACHOs could, at most, only constitute 10% of the accepted mass of our dark matter halo . Two years later, the EROS2 project extended this limit, concluding that any MACHO less than a single solar mass could not make up a significant part of the halo . The two collaborations were able to extend this to a rather large window, ruling out any MACHO in the 3.5 − 7 M ⊙ <m <30 M ⊙ (\ displaystyle 3.5 ^ (- 7) M_ (\ odot) <m <30M_ (\ odot)) window . The final window of extremely heavy MACHOs above 43 M ⊙ (\ displaystyle 43M_ (\ odot)) were ruled out by comparing Monte Carlo method simulations to observed distributions, which the authors indicate as "The End of the MACHO Era". Extremely light MACHOs less than the current limit are also not a viable possibility, as such light mass objects would not have survived on the timescale that it would take for the galaxy to form . </P> <P> The visible disk of the Milky Way Galaxy is embedded in a much larger, roughly spherical halo of dark matter . The dark matter density drops off with distance from the galactic center . It is now believed that about 95% of the Galaxy is composed of dark matter, a type of matter that does not seem to interact with the rest of the Galaxy's matter and energy in any way except through gravity . The luminous matter makes up approximately 9 x 10 solar masses . The dark matter halo is likely to include around 6 x 10 to 3 x 10 solar masses of dark matter . </P>

Why do astronomers think that there is a great amount of mass in the halo of the milky way
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