Recent discovery of high redshift Gamma-ray Bursts (GRBs) including
GRB090423 at z=8.2, the most distant object known so far, provides an
interesting new possibility to explore the early Universe in the Epoch
of Reionization (EOR). A next generation multi-wavelength observatory,
the Energy X-ray Imaging Survey Telescope (EXIST) is proposed to probe
the early Universe with high redshift GRBs, to survey black holes on all
scales, and to monitor the transient X-ray Universe. To accomplish this,
EXIST consists of three complementary telescopes; a High Energy Telescope
(HET), a Soft X-ray Imager (SXI), and an Optical/Infrared Telescope (IRT).
Here I will review the EXIST mission concept and the primary sciences. We
have been advancing the detector technology required for the HET through a
series of on-going balloon-borne experiments called ProtoEXIST. I will
show the result of the first balloon flight ProtoEXIST1 from Ft. Sumner,
New Mexico on 2010 Oct 9.