The atomic nucleus is made of protons and neutrons (nucleons), that are themselves composed of quarks and gluons. Understanding how the quark-gluon structure of a nucleon bound in an atomic nucleus is modified by the surrounding nucleons is an outstanding challenge. Although evidence for such modification, known as the EMC effect, was first observed over 35 years ago, there is still no generally accepted explanation of its cause."

Source :Nature, 556(7744), pp. 354-358.
Note: The details of the EMC effect are highly technical, and are described in this open-access paper.

Evidence of the EMC effect was discovered in data from experiments by the European Muon Collaboration (EMC) at CERN in 1983, and since then the results have been confirmed and replicated - and more than 1,000 research articles have been published about it.

The paper cited above was published in 2019, and, to date, there is still no agreed explanation for the effect.

Further technical reading CERN Courier, 2013