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# Brown dwarf [[star]]s and similarly massive but relatively non-luminous objects. Astronomers have in fact invented a new name for a class of objects that include brown dwarf stars and other massive objects: Massive Compact Halo Objects, or MACHOs.<ref name=Silk/><ref name=Miller/><ref name=WMAP2/><ref name=White>White, Martin. "[http://astro.berkeley.edu/~mwhite/darkmatter/dm.html Dark Matter]." Department of Astronomy, University of California at Berkeley, Berkeley, California, ca. 1995. Accessed July 28, 2008.</ref>

# Brown dwarf [[star]]s and similarly massive but relatively non-luminous objects. Astronomers have in fact invented a new name for a class of objects that include brown dwarf stars and other massive objects: Massive Compact Halo Objects, or MACHOs.<ref name=Silk/><ref name=Miller/><ref name=WMAP2/><ref name=White>White, Martin. "[http://astro.berkeley.edu/~mwhite/darkmatter/dm.html Dark Matter]." Department of Astronomy, University of California at Berkeley, Berkeley, California, ca. 1995. Accessed July 28, 2008.</ref>

# Supermassive [[black hole]]s. Astronomers are now attempting to detect these objects by their relativistic effects on light, in which they act as lenses.<ref name=WMAP2/>

# Supermassive [[black hole]]s. Astronomers are now attempting to detect these objects by their relativistic effects on light, in which they act as lenses.<ref name=WMAP2/>

# New, previously unknown forms of matter. Many cosmologists have formed hypotheses that suggest entirely new particles of matter. They call these Weakly Interactive Massive Particle, or WIMPs.<ref name=WMAP2/><ref name=Silk/><ref name=Miller/> Other cosmologists have suggested other types of particles, named ''axions''.<ref name=Hartnett/><ref name=axionnote>The name ''Axion'' is a registered trademark of the Colgate-Palmolive Company ([[USA]]) and was the name of a once-popular brand of laundry detergent used to pre-soak heavily-soiled garments before washing them with a conventional detergent. The astrophysicists who coined this name suggested that axions performed some kind of cosmic cleansing.</ref> The recently sought Higgs boson is another candidate for a dark-matter elementary particle.

# New, previously unknown forms of matter. Many cosmologists have formed hypotheses that suggest entirely new particles of matter. They call these Weakly Interactive Massive Particle, or WIMPs.<ref name=WMAP2/><ref name=Silk/><ref name=Miller/> Other cosmologists have suggested other types of particles, named ''axions''.<ref name=Hartnett/><ref name=axionnote>The name ''Axion'' is a registered trademark of the Colgate-Palmolive Company ([[USA]]) and was the name of a once-popular brand of laundry detergent used to pre-soak heavily-soiled garments before washing them with a conventional detergent. The astrophysicists who coined this name suggested that axions performed some kind of cosmic cleansing.</ref> The recently sought Higgs boson is another proposed dark-matter elementary particle.

# A new theory of gravity. In 1983, Mordecai Milgrom first suggested that Newtonian dynamics was insufficient to explain the gravitational interactions of massive objects like galaxies and galactic clusters. He therefore suggested a Modified Newtonian Dynamic, or MOND, in which gravitational attraction varied inversely to the first power of the orbital radius, not its square as Newton originally assumed.<ref name=Hartnett/><ref name=Thompson/>

# A new theory of gravity. In 1983, Mordecai Milgrom first suggested that Newtonian dynamics was insufficient to explain the gravitational interactions of massive objects like galaxies and galactic clusters. He therefore suggested a Modified Newtonian Dynamic, or MOND, in which gravitational attraction varied inversely to the first power of the orbital radius, not its square as Newton originally assumed.<ref name=Hartnett/><ref name=Thompson/>

Tim Thompson<ref name=Thompson/> has recently suggested yet another possibility to which most astronomers pay scant attention. He suggests that the major attractive force that allows galaxies and systems of higher mass to rotate with such excessive speed is not gravity at all, but electrostatic forces. He reminds his readers that electrostatic forces are stronger than gravity, and also that the strength of a magnetic field varies inversely as the first power, not the square, of the distance from the center. This is very close to Milgrom's MOND, with the advantage of having an underlying theory to explain it,<ref name=Thompson/> which Milgrom's system does not have.<ref name=Hartnett/>

=== Criticisms of the dark-matter concept ===

 

Don DeYoung challenged the notion of dark matter as a fanciful concept with little justification.<ref name=deYoung/> He pointed out that none of the conventional explanations popular at the time were satisfactory:

Don DeYoung challenged the notion of dark matter as a fanciful concept with little justification.<ref name=deYoung/> He pointed out that none of the conventional explanations popular at the time were satisfactory:

# Non-luminous stars, the usual candidates for MACHOs, would have to be far more common than they actually are, by several orders of magnitude, for them to account for the mass deficit.

# Non-luminous stars, the usual candidates for MACHOs, would have to be far more common than they actually are, by several orders of magnitude, for them to account for the mass deficit.

DeYoung also challenged the notion that galaxies or galactic clusters were necessarily stable. He did not comment directly on Milgrom's modified dynamic, but he did suggest that gravity was poorly understood.

DeYoung also challenged the notion that galaxies or galactic clusters were necessarily stable. He did not comment directly on Milgrom's modified dynamic, but he did suggest that gravity was poorly understood.

== References ==

== References ==