Some astronomers searching for what the mysterious “dark matter” might be made of have pinned their hopes on MACHO’s (MAssive Compact Halo Objects). What do they think these MACHO’s are?

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Starting with the question: astronomers consider MACHOs (Massive Compact Halo Objects) as potential constituents of dark matter. We will evaluate each answer choice to see which aligns with the MACHO concept, while also identifying why the alternatives do not fit.

Option a: 'large groups of college football players who have flunked astronomy, and now hang around the outskirts of town, just showing off their muscles.' This is clearly a humorous and fictional scenario with no basis in astronomy or physics. It does not describe any real astrophysical objects or mechanisms related to dark matter, so it is not a plausible candidate for MACHOs.

Option b: 'black holes, brown dwarfs, and white dwarfs in the regions outside the main disk of our Galaxy.' This choice lists compact, non-luminous or faint objects that could reside in the galactic halo. In MACHO hypotheses, such compact objects—particularly in the halo—are proposed as contributors to dark matter through gravitational effects, even though their exact contribution is debated. This option directly aligns with the standard MACHO concept.

Option c: 'huge concentrations of antimatter, outside of galaxies.' While antimatter is a legitimate physical concept, large, stable concentrations of antimatter outside galaxies are not supported by observations or cosmological models. Annihilation with ambient matter would produce gamma rays, which we do not see in the quantities this idea would require. Hence, this is not a MACHO explanation.

Option d: 'cannibal galaxies that have swallowed smaller galaxy neighbors until they have grown very large.' The idea of cannibal galaxies describes a process of hierarchical galaxy growth, not a population of compact halo objects. Although galaxy interactions are real, this description does not correspond to MACHOs, which are compact, stellar-moss-like remnants or similar compact bodies capable of hiding mass in halos.

Option e: 'vast clouds of neutrinos, emitted by ancient supernovae.' Neutrinos are light and weakly interacting, but the standard MACHO concept concerns compact, non-luminous or faint massive bodies that can contribute to dark matter through gravity. Pure neutrino seas would behave differently (and current data constrain light, hot dark matter components). Therefore, this does not reflect MACHO candidates.

In summary, the options present a mix of humorous, purely speculative, or physically inconsistent ideas versus a set that includes compact, long-lived objects that could reside in galactic halos. The most consistent MACHO interpretation involves compact stellar remnants or similar dense bodies located in the galactic halo, rather than exotic antimatter concentrations, galactic cannibalism scenarios, or diffuse neutrino backgrounds. All other choices fail to align with the MACHO concept for dark matter in the halo in a physically plausible way.

SC/NATS 1740 A - Astronomy (Summer 2025) Quiz 21

Some astronomers searching for what the mysterious “dark matter” might be made of have pinned their hopes on MACHO’s (MAssive Compact Halo Objects). What do they think these MACHO’s are?

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