= Solution
The primordial-element abundances from <Big Bang nucleosynthesis>, together with the <cosmic microwave background>, constrain the total baryonic density. <massive compact halo objects> such as ordinary faint stars, <brown dwarfs> and <stellar remnants> are baryonic whether or not they shine. With the stipulated density fractions, \b[the entire baryonic budget is only one fifth of the required dark-matter density], and much of it is already accounted for by ordinary gas and stars.
A second test does not require a compact object to emit light: <gravitational microlensing> temporarily magnifies a background star as the object crosses the line of sight. Monitoring many Magellanic-Cloud stars constrains the <microlensing optical depth> and event-duration distribution. Surveys such as EROS-2 found too few suitable events for a standard halo predominantly composed of compact objects over their tested <mass> range.<Limits on the Macho Content of the Galactic Halo from the EROS-2 Survey of the Magellanic Clouds by the EROS-2 Collaboration>[Abstract and halo-fraction limits; mass-dependent detection efficiency]{ref} Artificial-event injection, cadence and blending corrections are needed because very short or long events are easily missed; lensing by stars within the Clouds must also be separated from halo lensing. \b[These are mass-range and halo-model constraints, not a proof excluding every compact-object <mass>.] Primordial <black holes> are not limited by the ordinary-baryon budget in the same way as <stellar remnants>; their viability requires their own observational bounds.
Back to article page
