cosmologically distant quasars. One strong argument against them was that they implied energies that were far in excess of known energy conversion processes, including nuclear fusion. There were suggestions that quasars were made of some hitherto unknown stable form of antimatter in similarly unknown types of region of space, and that this might account for their brightness. Others speculated that quasars were a white hole end of a wormhole, or a chain reaction of numerous supernovae. Eventually, starting from about the 1970s, many lines of evidence (including the first X-ray space observatories, knowledge of black holes and modern models of cosmology) gradually demonstrated that the quasar redshifts are genuine and due to the expansion of space, that quasars are in fact as powerful and as distant as Schmidt and some other astronomers had suggested, and that their energy source is matter from an accretion disc falling onto a supermassive black hole. This included crucial evidence from optical and X-ray viewing of quasar host galaxies, finding of "intervening" absorption lines, which explained various spectral anomalies, observations from gravitational lensing, Gunn's 1971 finding that galaxies containing quasars showed the same redshift as the quasars, and Kristian's 1973 finding that the "fuzzy" surrounding of many quasars was consistent with a less luminous host galaxy. This model also fits well with other observations suggesting that many or even most galaxies have a massive central black hole. It would also explain why quasars are more common in the early universe: as a quasar draws matter from its accretion disc, there comes a point when there is less matter nearby, and energy production falls off or ceases, as the quasar becomes a more ordinary type of galaxy. The accretion-disc energy-production mechanism was finally modeled in the 1970s, and black holes were also directly detected (including evidence showing that supermassive black holes could be found at the centers of this and many other galaxies), which resolved the concern that quasars were too luminous to be a result of very distant objects or that a suitable mechanism could not be confirmed to exist in nature. By 1987 it was "well accepted" that this was the correct explanation for quasars, and the cosmological distance and energy output of quasars was accepted by almost all researchers. Modern observations (1970s onward) Later it was found that not all quasars have strong radio emission; in fact only about 10% are "radio-loud". Hence