They hope to be surer of that fact after a couple of years more data.
From ScienceDaily:
It all started with the measurements of the Planck satellite, which was launched by the European Space Agency (ESA) to measure the cosmic background radiation. This radiation is, to some extent, an afterglow of the big bang. It conveys crucial information on the matter distribution in the early universe; showing the distribution as it was only 380.000 years after the big bang.
According to the Planck measurements, this initial distribution was such that, over cosmic time, more galaxy clusters should have formed than we observe today. “We have measured with an X-ray satellite the number of galaxy clusters at different distances from ourselves,” explains Dr. Pacaud. The idea behind it: the light from remote galaxy clusters has traveled for billions of years before reaching us, and so we observe them today as they were when the universe was still young. Nearby clusters, on the contrary, are observed as they appeared much more recently.
“Our measurements confirm that the clusters formed too slowly,” said Dr. Pacaud. “We have estimated to which extent this result conflicts with the basic predictions of the standard model.” While there is a large discrepancy between the measurements and predictions, the statistical uncertainty in the present study is not yet tight enough to really put into question the theory. However, the researchers expect to obtain substantially more constraining results from the same project within the next three years. This will finally reveal whether the standard model needs to be revised. Paper. (open access) – F. Pacaud, M. Pierre, J.-.B. Melin, C. Adami, A. E. Evrard, F. Gastaldello, B. Maughan, M. Sereno. The XXL Survey. XXV. Cosmological analysis of the C1 cluster number counts. Astronomy & Astrophysics, 2018; DOI: 10.1051/0004-6361/201834022 More.
Given that the Standard Model (the Big Bang) is unpopular for philosophical reasons, one wonders whether any precision will ever put the matter to rest. See The Big Bang: Put simply, the facts are wrong and What becomes of science when the evidence does not matter? for the politics behind the problem.
As for the discrepancies, aw, not to worry just now. Everything seems so up in the air these days anyway. Consider:
See also: New Scientist on the glitch at the edge of the universe
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