What a creation! Darwin Centre's new wing is both a mystery and a triumph of design

It is described by its architects as a cocoon. Actually, it is more like a gargantuan pebble, or even the smoothly plastered nose of a Jumbo jet encased, much like Damien Hirst's shark, in a vast glass display case in London's Cromwell Road.

How apt that the new £78m wing of the Darwin Centre at the Natural History Museum, which opens to the public on 15 September, manages to suggest something evolving yet not quite identifiable. But that's precisely the point of this surprisingly self-effacing building, whose carefully tailored minimalism is not so much striking as pale and interesting.

The museum's Danish architects, CF Moller, may speak confidently of their new "icon" in South Kensington but, thankfully, it is nothing of the kind. Compared to the museum's original 2002 extension, the new wing stands deferentially next to the ornate haunch of its Grade I-listed Victorian terracotta tower. Inside, deference evaporates in a faintly surreal realm, in which potential architectural melodrama is coolly redeemed by the matt ivory stucco lucida covering the 60m-long pebble.

This host organism for the new wing's exhibition spaces, rising 30m up through the atrium, is both a mystery and a triumph: how can the biggest, curved sprayed-concrete structure in Europe look weightless?

There's nothing mysterious about the result, though – this is a new kind of science experience in which the pursuit of research becomes more or less public property.

"Many people love the Natural History Museum for its iconic Victorian building," said Neil Greenwood, programme director for the Darwin Centre. "However, through the Darwin Centre, we wanted to challenge this traditional perception and highlight the work of our scientists, and the importance of our collections."

Paul Bowers, director of the new wing's public spaces, added: "We wanted an exquisite shell that's beautiful to flow through."

The building's lead designer, Anna Maria Indrio, says that the new building "has completely changed the Natural History Museum's relationship with the site, from being an introvert to an extrovert building." Surely not. Alfred Waterhouse's original architecture has always been a riveting and distinctly outré mélange of star turns and engrossing detail. There is absolutely nothing shy about the 19th-century architecture, or its interiors.

Ms Indrio is, however, right to suggest that images of the pebble will become a new brandmark for the museum. But as the building's pale, set-piece architectural moment can be seen only faintly through the glass façade of the atrium, this brandmark is anything but extrovert architecture – and all the better for it.

The Darwin Centre's new wing was designed at the same time as the doomed V&A Spiral, created by Daniel Libeskind and Cecil Balmond.

Unlike the architecturally radical Spiral, Moller's "quiet" architecture managed to attract the necessary funding from the Heritage Lottery, the Wellcome Trust and a phalanx of private donors.

The simplicity of the pebble and its exhibition route is matched by the new clarity it brings to the way the Darwin Centre's 220 scientists can now move between the old and new buildings, and into a doubled amount of laboratory space.

From next Tuesday, inquisitive members of the public will be able to rise seven storeys in sparkling glass lifts to get into the pebble, before descending in batches of about 200 at a time to follow a trail of discovery laid out for them inside.

They will glimpse a fraction of the 20 million entomological and botanical specimens filed away in 3.3km of temperature and humidity controlled "compaction" cabinets in the lower segment of the structure. But they will see scientists at work and, sometimes, be able to quiz tweezering boffins through glass panels fitted with two-way microphones.

Somehow, given the contemporary trend to present art or science as a blur of faintly trivial entertainment, the Darwin Centre's new wing is not packed with interactive kit. On the other hand, it is well chosen, generously spaced out, and mounted in such a way that it conveys a sense of seriously pursued discovery – unlike the new wing's Attenborough Studio, a vividly over-the-top lecture space whose pentagram-designed fibreglass seating evokes a Star Wars briefing room.