  The EXperiment for Cryogenic Large-Aperture Intensity Mapping (EXCLAIM) is a
balloon-borne far-infrared telescope that will survey star formation history
over cosmological time scales to improve our understanding of why the star
formation rate declined at redshift z < 2, despite continued clustering of dark
matter. Specifically,EXCLAIM will map the emission of redshifted carbon
monoxide and singly-ionized carbon lines in windows over a redshift range 0 < z
< 3.5, following an innovative approach known as intensity mapping. Intensity
mapping measures the statistics of brightness fluctuations of cumulative line
emissions instead of detecting individual galaxies, thus enabling a blind,
complete census of the emitting gas. To detect this emission unambiguously,
EXCLAIM will cross-correlate with a spectroscopic galaxy catalog. The EXCLAIM
mission uses a cryogenic design to cool the telescope optics to approximately
1.7 K. The telescope features a 90-cm primary mirror to probe spatial scales on
the sky from the linear regime up to shot noise-dominated scales. The telescope
optical elements couple to six {\mu}-Spec spectrometer modules, operating over
a 420-540 GHz frequency band with a spectral resolution of 512 and featuring
microwave kinetic inductance detectors. A Radio Frequency System-on-Chip
(RFSoC) reads out the detectors in the baseline design. The cryogenic telescope
and the sensitive detectors allow EXCLAIM to reach high sensitivity in spectral
windows of low emission in the upper atmosphere. Here, an overview of the
mission design and development status since the start of the EXCLAIM project in
early 2019 is presented.
