1. See, for example, V. Trimble, Ann. Rev. Astron. Astrophys. 25 (1987) 425; J.R. Primack, D. Seckel, B. Sadoulet, Ann. Rev. Nucl. Part. Sci. 38 (1988) 751; S. Tremaine, Physics Today 45 (1992) 28.
2. C. Alcock et al., Nature 365 (1993) 621; E. Aubourg et al., Nature 365 (1993) 623; A. Udalski et al., Acta Astronomica 43 (1993) 289; C. Alcock et al., astro-ph/9606165.
3. E.I. Gates, G. Gyuk, G.P. Holder, M.S. Turner, FERMILAB-PUB-97-381-A, astro-ph/9711110.
4. M. White, D. Scott, J. Silk, Ann. Rev. Astron. Astrophys. 32 (1994) 319.
5. See, for example, J.R. Primack, astro-ph/9707285.