Speaking as another layman, who has done plenty of reading and checking out the most reputable sites, DM is on reasonable solid grounds.
The following was fairly convincing evidence that it does exist.
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"It's a bet that finally paid off, because a team of scientists working with NASA's Chandra X-ray Observatory has found direct evidence that dark matter is as real as the rings around Saturn.
The discovery cements dark matter's status as the biggest building block in the universe, while also putting to rest the nagging worries of many astronomers that they gambled wrong.
Dark matter's murky nature has always sat a bit uneasily with astronomers. "It is uncomfortable for a scientist to have to invoke something invisible and undetectable to account for 90 percent of the matter in the universe," said Maxim Markevitch, a Chandra astrophysicist and researcher with the study.
more at.....
http://www.nasa.gov/vision/universe/starsgalaxies/dark_matter_proven.html
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I also had an astronomer once tell me we had more evidence for DM then we do for BH's.
He though possibily may have been carrying some baggage, as this was his particular field of expertise.
DE on the other hand is a complete mystery. It has been surmised that it is the Cosmological Constant [Albert's greatest blunder] or a type of quintessence.
But we must remember that cosmologists did not just pull the DE concept out of their arse. It was proposed after data from WMAP was researched and surprised them as much as it did ordinary lay people like you and me. It was a complete necessity to align with what had been observed....that is acceleration in the expansion rate.
To revisit GR for modification is really the last option, as it has so much backing and support, by so much observational data [GP-B being the latest confirmation] that tinkering cosmologists would be rightly reluctant to tinker with such well supported confirmed data.