Evidence of the clouds had been previously detected around our own galaxy and in the Local Group, but the temperature range (a few hundred thousand to a million degrees Celsius) and low density have prevented detection outside the immediate neighborhood of galaxies and the reliable estimation of baryon density for the entire universe.
雲の存在証拠は我々自身の銀河の周りと局部群の中に予てより発見されていたが、温度領域（摂氏数10万から100万度）と低密度であることが銀河直近外部の探査、及び宇宙全体の重粒子密度の信頼性のある見積もりを阻んできた。