Dark matter will be discussed by John Orrell, a Pacific Northwest National Laboratory scientist, on Tuesdayin Richland.

He will discuss how we know dark matter exists, what properties can be inferred by the lack of measurements to date and why dark matter is relevant to our understanding of the cosmos.

He will speak at a meeting of the Eastern Washington Section of the American Nuclear Society at 7 p.m. at the Shilo Inn, Richland. There is no charge for attending his talk, but those who attend the dinner meeting an hour earlier will be charged $22 and should make reservations by 4 p.m. Fridayat ANS.EWS.Treasurer@gmail.com.