Vacuum
The vacuum is space from which all matter has been removed — and, in modern physics, the lowest-energy state of the fields that remain when everything removable has been taken away. The two halves of that sentence are not the same thing, and the gap between them is one of the most consequential unsettled questions in physics.
The standard account
In classical physics a vacuum is simply emptiness: no particles, no pressure, nothing to carry a force. Laboratory vacuum is always partial — the best ultra-high-vacuum chambers reach around 10-10 pascal, and interstellar space still holds roughly one hydrogen atom per cubic centimetre — but the idealisation of a perfectly empty container was central to Newtonian mechanics and to the nineteenth-century debate over whether light needed a medium. The Michelson-Morley result of 1887 (see Michelson-Morley experiment) and Einstein's 1905 paper were together taken to have removed the last reason to think space contained anything at all.
Quantum field theory then put a great deal back. In QFT the vacuum is the ground state of the quantum fields, and that ground state is not featureless: each field mode retains a zero-point energy of hν/2, and the vacuum has measurable consequences. The Lamb shift (1947), the anomalous magnetic moment of the electron, vacuum polarisation corrections in quantum electrodynamics, and the attraction between uncharged conducting plates predicted by Casimir in 1948 and measured by Lamoreaux in 1997 are all standardly explained as effects of vacuum structure. In quantum chromodynamics the vacuum carries a gluon and quark condensate; in the electroweak theory it carries the Higgs field. The vacuum, in other words, has a permittivity, a permeability, a polarisability, and phase transitions.
The unresolved problem is what all this weighs. Summing the zero-point energies of the fields gives an energy density enormously larger than the value inferred from cosmological observation — the mismatch is commonly quoted as many tens of orders of magnitude, and in the most naive estimate around 120. This is the cosmological constant problem, and it is not a dissident complaint: it is openly acknowledged in mainstream literature as an unexplained discrepancy.
On this wiki
The researchers catalogued here read that situation as a rehabilitation of the aether under another name. The clearest statement is Michele Barone's The Vacuum as Ether in the last Century (Foundations of Physics, 2004), which reviews the vacuum as it appears in quantum mechanics, QED, QCD and cosmology and concludes that in every one of them it is "a 'fluid' made up by matter and radiation present in the whole Universe, which may be identified with a modern definition of ether." Barone makes the argument entirely from orthodox physics, disputing no calculation — the dissident content is the conclusion that a medium with polarisation, pressure, phase transitions and a rest state is an aether by any reasonable definition, and that its rejection was premature rather than final.
A second, larger thread treats the vacuum's energy as a potential energy source rather than a bookkeeping term. Harold E Puthoff is the central figure: The Energetic Vacuum: Implications for Energy Research (1990), Quantum Fluctuations of Empty Space: A New Rosetta Stone of Physics? (1991), and — with Bernard Haisch and Alfonso Rueda — Physics of the Zero-Point Field: Implications for Inertia, Gravitation and Mass (1997), which argues that inertia itself may be a reaction force originating in the zero-point field rather than an intrinsic property of matter. Thomas F Valone (Understanding Zero Point Energy, Practical Conversion of Zero Point Energy: Feasibility Study of the Extraction of Zero Point Energy from the Quantum Vacuum for the Performance of Useful Work), Moray B King (Quest for Zero Point Energy Engineering Principles for Free Energy, Vacuum Energy Vortices) and Friedwardt Winterberg (Wheeler's Geometrodynamics and the Zero Point Vacuum Energy) work this vein; see Category:Zero Point Energy and Category:New Energy. Extraction claims in this literature range from careful theoretical feasibility studies to device claims that have not been independently replicated, and the wiki catalogues both — Thomas F Valone's own A Critical Review of the Available Information Regarding Claims of Zero-Point Energy, Free-Energy, and Over-Unity Experiments and Devices is a sceptical survey from inside the field.
A third line takes the vacuum's gravitational behaviour seriously. Dragan Slavkov Hajdukovic proposes that virtual particle–antiparticle pairs act as gravitational dipoles, so that gravitational polarisation of the vacuum by ordinary matter would mimic dark matter — a hypothesis that depends on antimatter having negative gravitational mass, which remains untested.